Biomonitoring: Perchlorate
Methods
Indicator
B13. Perchlorate in women ages 16 to 49 years: Median and 95th percentile concentrations in
urine, 2001-2014
Summary
Since the 1970s, the National Center for Health Statistics, a division of the Centers for Disease
Control and Prevention, has conducted the National Health and Nutrition Examination Surveys
(NHANES), a series of U.S. national surveys of the health and nutrition status of the
noninstitutionalized civilian population. The National Center for Environmental Health at CDC
measures environmental chemicals in blood and urine samples collected from NHANES
participants.1 This indicator uses urine measurements of perchlorate in women ages 16 to 49
years and children ages 6 to 17 years. The NHANES 2001-2002, 2003-2004, 2005-2006, 2007-
2008, 2009-2010, 2011-2012, and 2013-2014 surveys included urine perchlorate data for
children and adults ages 6 years and over.
Indicator B13 is the trend in the median and 95th percentile concentrations of perchlorate in
women ages 16 to 49 years for 2001-2014. The median is the estimated concentration such that
50% of all noninstitutionalized civilian women ages 16 to 49 years during the survey period have
a perchlorate concentration below this level; the population distribution was adjusted by age-
specific birth rates to reflect exposures to women who are pregnant or may become pregnant.
The 95th percentile is the estimated concentration such that 95% of all noninstitutionalized
civilian women ages 16 to 49 years during the survey period have a perchlorate concentration
below this level. Table B13a presents the median concentration of perchlorate for women ages
16 to 49 years for 2011-2014, stratified both by race/ethnicity and family income. Table B13b
presents the 95th percentile concentration of perchlorate for women ages 16 to 49 years for 2011-
2014, stratified both by race/ethnicity and family income. Table B13c presents the trend in the
median and 95th percentile concentrations of perchlorate for children ages 6 to 17 years for 2001-
2014. Table B13d presents the median concentration of perchlorate for children ages 6 to 17
years in 2011-2014, stratified both by race/ethnicity and family income. Table B13e presents the
95th percentile concentration of perchlorate for children ages 6 to 17 years in 2011-2014,
stratified both by race/ethnicity and family income. Table B13f presents the median and 95th
percentile concentrations of perchlorate for children ages 6 to 17 in 2011-2014, stratified by age.
The survey data were weighted to account for over-sampling, non-response, and non-coverage.
1 Centers for Disease Control and Prevention. 2009. Fourth National Report on Human Exposure to Environmental
Chemicals. Atlanta. GA. Available at: www.cdc.gov/exposurereport.
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Data Summary
Indicator
Indicator B13. Perchlorate in women ages 16 to 49 years:
Median and 95th percentile concentrations in urine, 2001-
2014.
Time Period
2001-2014
Data
Urine perchlorate in women ages 16 to 49 years
Years
2001-2002
2003-2004
2005-2006
2007-2008
Limits of Detection (|ig/L)*
0.05
0.05
0.05
0.05
Number of values
700
623
2085
1749
Number of Non-missing
Values**
657 (94%)
616 (99%)
1921 (92%)
1608 (92%)
Number of Missing Values**
43 (6%)
7 (1%)
164 (8%)
141 (8%)
Percentage Below Limit of
Detection***
0
0
0
0
Years
2009-2010
2011-2012
2013-2014
Limits of Detection (|ig/L)*
0.05
0.05
0.05
Number of values
686
542
632
Number of Non-missing
Values**
662 (97%)
528 (97%)
610 (97%)
Number of Missing Values**
24 (3%)
14 (3%)
22 (3%)
Percentage Below Limit of
Detection***
0
0
0
* The Limit of Detection (LOD) is defined as the level at which the measurement has a 95% probability of being greater than
zero.
**Non-missing values include those below the analytical LOD, which are reported as LOD/ V2. Missing values are the number of
sampled women ages 16 to 49 years in the Mobile Examination Center (MEC) sub-sample that have no value reported for the
particular variable used in calculating the indicator.
** *This percentage is survey-weighted using the NHANES MEC survey weights for the given period and is weighted by age-
specific birth rates.
Indicator
Indicator B13. Perchlorate in women ages 16 to 49 years:
Median and 95th percentile concentrations in urine, 2001-2008
Time Period
2001-2008
Data
Urine perchlorate in children ages 6 to 17 years.
Years
2001-2002
2003-2004
2005-2006
2007-2008
Limits of Detection (|ig/L)*
0.05
0.05
0.05
0.05
Number of values
1058
859
2849
2207
Number of Non-missing
Values**
1021 (97%)
858 (100%)
2626 (92%)
2012 (91%)
Number of Missing Values**
37 (3%)
1 (0%)
223 (8%)
195 (9%)
Percentage Below Limit of
Detection***
0
0
0
0
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Indicator
Indicator B13. Perchlorate in women ages 16 to 49 years:
Median and 95th percentile concentrations in urine, 2001-2008
Years
2009-2010
2011-2012
2013-2014
Limits of Detection (|ig/L)*
0.05
0.05
0.05
Number of values
769
706
775
Number of Non-missing
Values**
734 (95%)
683 (97%)
728 (94%)
Number of Missing Values**
35 (5%)
23 (3%)
47 (6%)
Percentage Below Limit of
Detection***
* The Limit of Detection (LOD) is defined as the level at which the measurement has a 95% probability of being greater than
zero.
**Non-missing values include those below the analytical LOD, which are reported as LOD/ V2. Missing values are the number of
sampled children ages 6 to 17 years in the Mobile Examination Center (MEC) sub-sample that have no value reported for the
particular variable used in calculating the indicator.
** *This percentage is survey-weighted using the NHANES MEC survey weights for the given period.
Overview of Data Files
The following files are needed to calculate this indicator. The files together with the survey
documentation and SAS programs for reading in the data are available at the NHANES website:
http://www.cdc.gov/nchs/nhanes/nhanes questionnaires.htm.
• NHANES 2001-2002: Demographic file demo_b.xpt. Nitrate, thiocyanate, perchlorate
(Surplus Urine) laboratory file ssno3p_b.xpt. The demographic file demo b.xpt is a SAS
transport file that contains the subject identifier (SEQN), age (RIDAGEYR), sex
(RIAGENDR), race/ethnicity (RIDRETH1), poverty income ratio (INDFMPIR), pseudo-
stratum (SDMVSTRA) and the pseudo-PSU (SDMVPSU). The Nitrate, thiocyanate,
perchlorate (Surplus Urine) laboratory file ssno3p_b.xpt contains SEQN, urine
perchlorate (SSXUP8), and the two year Mobile Examination Center (MEC) sub-sample
weight (WTUI02YR). The two files are merged using the common variable SEQN.
• NHANES 2003-2004: Demographic file demo_c.xpt. Urinary Perchlorate laboratory file
104per_c.xpt. The demographic file demo c.xpt is a SAS transport file that contains the
subject identifier (SEQN), age (RIDAGEYR), sex (RIAGENDR), race/ethnicity
(RIDRETH1), poverty income ratio (INDFMPIR), pseudo-stratum (SDMVSTRA) and
the pseudo-PSU (SDMVPSU). The Urinary Perchlorate laboratory file 104per_c.xpt
contains SEQN, urine perchlorate (URXUP8), and the two year MEC sub-sample C
weight (WTSC2YR). The two files are merged using the common variable SEQN.
• NHANES 2005-2006: Demographic file demo_d.xpt. Urinary Nitrate, Urinary
Perchlorate, Urinary Thiocyanate laboratory file pernt d.xpt. The demographic file
demo d.xpt is a SAS transport file that contains the subject identifier (SEQN), age
(RIDAGEYR), sex (RIAGENDR), race/ethnicity (RIDRETH1), poverty income ratio
(INDFMPIR), pseudo-stratum (SDMVSTRA), pseudo-PSU (SDMVPSU), and the two
year MEC weight (WTMEC2YR). The Urinary Nitrate, Urinary Perchlorate, Urinary
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Thiocyanate laboratory file pernt d.xpt contains SEQN, urine perchlorate (URXUP8),
and the perchlorate non-detect comment code (URDUP8LC). The two files are merged
using the common variable SEQN.
• NHANES 2007-2008: Demographic file demo_e.xpt. Urinary Nitrate, Urinary
Perchlorate, Urinary Thiocyanate laboratory file pernte.xpt. The demographic file
demo e.xpt is a SAS transport file that contains the subject identifier (SEQN), age
(RIDAGEYR), sex (RIAGENDR), race/ethnicity (RIDRETH1), poverty income ratio
(INDFMPIR), pseudo-stratum (SDMVSTRA), pseudo-PSU (SDMVPSU), and the two
year MEC weight (WTMEC2YR). The Urinary Nitrate, Urinary Perchlorate, Urinary
Thiocyanate laboratory file pernt e.xpt contains SEQN, urine perchlorate (URXUP8),
and the perchlorate non-detect comment code (URDUP8LC). The two files are merged
using the common variable SEQN.
• NHANES 2009-2010: Demographic file demo_f.xpt. Perchlorate, Nitrate & Thiocyanate
- Urine laboratory file pernt f.xpt. The demographic file demof.xpt is a SAS transport
file that contains the subject identifier (SEQN), age (RIDAGEYR), sex (RIAGENDR),
race/ethnicity (RIDRETH1), poverty income ratio (INDFMPIR), pseudo-stratum
(SDMVSTRA), pseudo-PSU (SDMVPSU), and the two year MEC sub-sample weight
(WTSA2YR). Perchlorate, Nitrate & Thiocyanate - Urine laboratory file perntf xpt
contains SEQN, urine perchlorate (URXUP8), and the perchlorate non-detect comment
code (URDUP8LC). The two files are merged using the common variable SEQN.
• NHANES 2011-2012: Demographic file demof xpt. Perchlorate, Nitrate & Thiocyanate
- Urine laboratory file pernt g.xpt. The demographic file demo g.xpt is a SAS transport
file that contains the subject identifier (SEQN), age (RIDAGEYR), sex (RIAGENDR),
race/ethnicity (RIDRETH1), poverty income ratio (INDFMPIR), pseudo-stratum
(SDMVSTRA), pseudo-PSU (SDMVPSU), and the two year MEC subsample weight
(WTSA2YR). Perchlorate, Nitrate & Thiocyanate - Urine laboratory file pernt g.xpt
contains SEQN, urine perchlorate (URXUP8), and the perchlorate non-detect comment
code (URDUP8LC). The two files are merged using the common variable SEQN.
• NHANES 2013-2014: Demographic file demo_h.xpt. Perchlorate, Nitrate & Thiocyanate
- Urine laboratory file pernth.xpt. The demographic file demo h.xpt is a SAS transport
file that contains the subject identifier (SEQN), age (RIDAGEYR), sex (RIAGENDR),
race/ethnicity (RIDRETH1), poverty income ratio (INDFMPIR), pseudo-stratum
(SDMVSTRA), pseudo-PSU (SDMVPSU), and the two year MEC sub-sample weight
(WTSA2YR). Perchlorate, Nitrate & Thiocyanate - Urine laboratory file pernt h.xpt
contains SEQN, urine perchlorate (URXUP8), and the perchlorate non-detect comment
code (URDUP8LC). The two files are merged using the common variable SEQN.
National Health and Nutrition Examination Surveys (NHANES)
Since the 1970s, the National Center for Health Statistics, a division of the Centers for Disease
Control and Prevention, has conducted the National Health and Nutrition Examination Surveys
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(NHANES), a series of U.S. national surveys of the health and nutrition status of the
noninstitutionalized civilian population. The National Center for Environmental Health at CDC
measures environmental chemicals in blood and urine samples collected from NHANES
participants. This indicator uses urine perchlorate measurements from NHANES 2001-2002,
2003-2004, 2005-2006, 2007-2008. 2009-2010, 2011-2012, and 2013-2014 in women ages 16 to
49 and children ages 6 to 17. The NHANES data were obtained from the NHANES website:
http://www.cdc.gov/nchs/nhanes.htm. Following the CDC recommended approach, values below
the analytical limit of detection (LOD) were replaced by LOD/V2."
The NHANES use a complex multi-stage, stratified, clustered sampling design. Certain
demographic groups were deliberately over-sampled, including Mexican-Americans, Blacks,
and, from 2007 onwards, All Hispanics, to increase the reliability and precision of estimates of
health status indicators for these population subgroups. The publicly released data includes
survey weights to adjust for the over-sampling, non-response, and non-coverage. The statistical
analyses used the applicable MEC sample or sub-sample survey weights (WTUI02YR for 2001-
2002, WTSC2YR for 2003-2004, WTMEC2YR for 2005-2006 and 2007-2008, WTSA2YR for
2009-2010, 2011-2012, and 2013-2014) to re-adjust the urine perchlorate data to represent the
national population.
Age-Specific Birth Rates
In addition to the NHANES MEC survey weights, the data for women of child-bearing age (ages
16 to 49) were also weighted by the birth rate for women of the given age and race/ethnicity to
estimate prenatal exposures. Thus the overall weight in each two year period is the product of the
NHANES survey weight and the total number of births in the two calendar years for the given
age and race/ethnicity, divided by twice the corresponding population of women at the midpoint
of the two year period:111
Adjusted Survey Weight =
MEC survey weight x U.S. Births (NHANES cycle, age, race/ethnicity) /
{Number of years in NHANES cycle x U.S. Women (NHANES cycle midpoint, age,
race/ethnicity)}.
Race/Ethnicity and Family Income
For these indicators, the percentiles were calculated for demographic strata defined by the
combination of race/ethnicity and family income.
The family income was characterized based on the INDFMPIR variable, which is the ratio of the
family income to the poverty level. The National Center for Health Statistics used the U.S.
Census Bureau Current Population Survey definition of a "family" as "a group of two people or
II See Hornung RW, Reed LD. 1990. Estimation of average concentration in the presence of nondetectable values.
Applied Occupational and Environmental Hygiene 5:46-51.
III Axelrad, D.A., Cohen J. 2011. Calculating summary statistics for population chemical biomonitoring in women of
child-bearing age with adjustment for age-specific natality. Environmental Research 111 (1) 149-155.
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more (one of whom is the householder) related by birth, marriage, or adoption and residing
together" to group household members into family units, and the corresponding family income
for the respondent was obtained during the interview. The U.S. Census Bureau defines annual
poverty level money thresholds varying by family size and composition. The poverty income
ratio (PIR) is the family income divided by the poverty level for that family. Family income was
stratified into the following groups:
• Below Poverty Level: PIR < 1
• Above Poverty Level: PIR > 1
• Unknown Income: PIR is missing
For the four year period 2011-2012, the weighted percentage of women ages 16 to 49 years with
unknown income was 6% and the weighted percentage of children ages 6 to 17 years with
unknown income was 6%.
Race/ethnicity was characterized using the RIDRETH1 variable. The possible values of this
variable are:
• 1. Mexican American
• 2. Other Hispanic
• 3. Non-Hispanic White
• 4. Non-Hispanic Black
• 5. Other Race - Including Multi-racial
• Missing
Category 5 includes: all Non-Hispanic single race responses other than White or Black; and
multi-racial responses.
For this indicator, the RIDRETH1 categories 2, 5, and missing were combined into a single "All
Other Races/Ethnicities" category. This produced the following categories:
• White non-Hispanic: RIDRETH1 = 3
• Black non-Hispanic: RIDRETH1 = 4
• Mexican-American: RIDRETH1 = 1
• All Other Races/Ethnicities: RIDRETH1 = 2 or 5 or missing
The "All Other Races/Ethnicities" category includes multiracial persons and individuals whose
racial or ethnic identity is not White non-Hispanic, Black non-Hispanic, or Mexican-American.
Except for non-Mexican-American Hispanics in 2007-2014 and Asian non-Hispanics in 2011-
2014, persons of "All Other Races/Ethnicities" are selected into the survey with a probability
that is very much lower than White non-Hispanic, Black non-Hispanic and Mexican-American
individuals, and as a group they are not representative of all other race and ethnicities in the
United States.
Calculation of Indicator
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Indicator B13 is the median and 95th percentile for urine perchlorate in women of ages 16 to 49
years, stratified by NHANES survey cycle. The median for women ages 16 to 49 is the estimated
concentration such that 50% of all noninstitutionalized civilian women ages 16 to 49 years
during the survey period have urine perchlorate concentrations below this level. The 95th
percentile for women ages 16 to 49 is the estimated concentration such that 95% of all
noninstitutionalized civilian women ages 16 to 49 years during the survey period have urine
perchlorate concentrations below this level. To adjust the NHANES data to represent prenatal
exposures, the data for each woman surveyed was multiplied by the estimated number of births
per woman of the given age and race/ethnicity. Table B13a presents the median for urine
perchlorate in women of ages 16 to 49 years in 2011-2014, stratified by race/ethnicity and family
income. Table B13b presents the 95th percentile for urine perchlorate in women of ages 16 to 49
years in 2011-2014, stratified by race/ethnicity and family income. Table B13c presents the
median and 95th percentile for urine perchlorate in children of ages 6 to 17 years, stratified by
NHANES survey cycle. The birth rate adjustment was not applied to children ages 6 to 17. Table
B13d presents the median for urine perchlorate in children of ages 6 to 17 years in 2011-2014,
stratified by race/ethnicity and family income. Table B13e presents the 95th percentile for urine
perchlorate in children of ages 6 to 17 years in 2011-2014, stratified by race/ethnicity and family
income. Table B13f presents the median and 95th percentile concentration of perchlorate for
children ages 6 to 17 in 2011-2014, stratified by age.
To simply demonstrate the calculations, we will use the NHANES 2007-2008 urine perchlorate
values for women ages 16 to 49 years of all race/ethnicities and all incomes as an example. We
have rounded all the numbers to make the calculations easier:
We begin with all the non-missing NHANES 2007-2008 urine perchlorate values for women
ages 16 to 49 years. Assume for the sake of simplicity that valid perchlorate data were available
for every sampled woman. Each sampled woman has an associated annual survey weight that
estimates the annual number of U.S. women represented by that sampled woman. For 2007-
2008, the associated annual survey weight for each woman is defined as WTMEC2YR. Each
sampled woman also has an associated birth rate giving the numbers of annual births per woman
of the given age, race, and ethnicity. The product of the annual survey weight and the birth rate
estimates the annual number of U.S. births represented by that sampled woman, which we will
refer to as the adjusted survey weight. For example, the lowest urine perchlorate measurement
for a woman between 16 and 49 years of age is 0.1 |ig/L with an annual survey weight of 25,000,
a birth rate of 0.008, and thus an adjusted survey weight of 200, and so represents 200 births. The
total of the adjusted survey weights for the sampled women equals 4 million, the total number of
annual U.S. births to women ages 16 to 49 years. The second lowest measurement is also 0.1
|ig/L with an adjusted survey weight of 8, and so represents another 8 U.S. births. The highest
measurement is 220 |ig/L with an adjusted survey weight of 30, and so represents another 30
U.S. births.
To calculate the median, we can use the adjusted survey weights to expand the data to the entire
U.S. population of births to women ages 16 to 49. We have 200 values of 0.1 |ig/L from the
lowest measurement, 8 values of 0.1 |ig/L from the second lowest measurement, and so on, up to
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30 values of 220 |ig/L from the highest measurement. Arranging these 4 million values in
increasing order, the 2 millionth value is 3.4 |ig/L. Since half of the values are below 3.4 and half
of the values are above 3.4, the median equals 3.4 |ig/L. To calculate the 95th percentile, note that
95% of 4 million equals 3.8 million. The 3.8 millionth value is 16.5 |ig/L. Since 95% of the
values are below 16.5, the 95th percentile equals 16.5 |ig/L.
In reality, the calculations need to take into account that urine perchlorate measurements were
not available for every respondent, and to use exact rather than rounded numbers. There were
urine perchlorate measurements for only 1608 of the 1749 sampled women ages 16 to 49 years.
The adjusted survey weights for all 1749 sampled women add up to 4.2 million, the U.S.
population of births to women ages 16 to 49. The adjusted survey weights for the 1608 sampled
women with urine perchlorate data add up to 3.9 million. Thus the available data represent 3.9
million values and so represent only 94% of the U.S. population of births. The median and 95th
percentiles are given by the 1.95 millionth (50% of 3.9 million) and 3.7 millionth (95% of 3.9
million) U.S. birth's value. These calculations assume that the sampled women with valid urine
perchlorate data are representative of women giving birth without valid urine perchlorate data.
The calculations also assume that the sampled women are representative of women that actually
gave birth in 2007-2008, since NHANES information on pregnancy and births was not
incorporated into the analysis.
Equations
These percentile calculations can also be given as the following mathematical equations, which
are based on the default percentile calculation formulas from Statistical Analysis System (SAS)
software. Exclude all missing urine perchlorate values. Suppose there are n women of ages 16 to
49 years with valid urine perchlorate values. Arrange the urine perchlorate concentrations in
increasing order (including tied values) so that the lowest concentration is x(l) with an adjusted
survey weight of w(l), the second lowest concentration is x(2) with an adjusted survey weight of
w(2), ..., and the highest concentration is x(n) with an adjusted survey weight of w(n).
1. Sum all the adjusted survey weights to get the total weight W:
W = E[1 < i < n] w(i)
2. Find the largest number i so that the total of the weights for the i lowest values is less than or
equal to W/2.
E[j < i] w(j) < W/2 < E[j < i + 1] w(j)
3. Calculate the median using the results of the second step. We either have
E[j < i] w(j) = W/2 < E[j < i + 1] w(j)
or
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E[j < i] w(j) < W/2 < E[j < i + 1] w(j)
In the first case we define the median as the average of the i'th and i + l'th values:
Median = [x(i) + x(i + l)]/2 if S[j < i] w(j) = W/2
In the second case we define the median as the i + l'th value:
Median = x(i + 1) if E[j < i] w(j) < W/2
(The estimated median does not depend upon how the tied values of x(j) are ordered).
A similar calculation applies to the 95th percentile. The first step to calculate the sum of the
weights, W, is the same. In the second step, find the largest number i so that the total of the
weights for the i lowest values is less than or equal to 0.95W.
E[j < i] w(j) < 0.95W < E[j < i + 1] w(j)
In the third step we calculate the 95th percentile using the results of the second step. We either
have
E[j < i] w(j) = 0.95W < E[j < i + 1] w(j)
or
E[j < i] w(j) < 0.95W < E[j < i + 1] w(j)
In the first case we define the 95th percentile as the average of the i'th and i + l'th values:
95th Percentile = [x(i) + x(i + l)]/2 if S[j < i] w(j) = 0.95W
In the second case we define the 95th percentile as the i + l'th value:
95th Percentile = x(i + 1) if S[j < i] w(j) < 0.95W
Relative Standard Error
The uncertainties of the median and 95th percentile values were calculated using a revised
version of the CDC method given in CDC 2005,1V Appendix C, and the SAS® program provided
by CDC. The method uses the Clopper-Pearson binomial confidence intervals adapted for
complex surveys by Korn and Graubard (see Korn and Graubard, 1999,v p. 65). The following
lv CDC Third National Report on Human Exposure to Environmental Chemicals. 2005
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text is a revised version of the Appendix C. For the birth rate adjusted calculations for women
ages 16 to 49, the sample weight is adjusted by multiplying by the age-specific birth rate.
Step 1: Use SAS® Proc Univariate to obtain a point estimate Psas of the percentile value. Use the Weight
option to assign the exact correct sample weight for each chemical result.
Step 2: Use SUDAAN® Proc Descript with Taylor Linearization DESIGN = WR (i.e.,
sampling with replacement) and the proper sampling weight to estimate the proportion (p) of subjects with
results less than and not equal to the percentile estimate Psas obtained in Step 1 and to obtain the standard
error (sep) associated with this proportion estimate. Compute the degrees-of-freedom adjusted effective
sample size
n : — ( tnum/ tdenom) p ( 1 — P ) / ( SCp )
where trium and tdenom are 0.975 critical values of the Student's t distribution with degrees of freedom
equal to the sample size minus 1 and the number of PSUs minus the number of strata, respectively. Note:
the degrees of freedom for tdenom can vary with the demographic sub-group of interest.
Step 3: After obtaining an estimate of p (i.e., the proportion obtained in Step 2), compute the Clopper-
Pearson 95% confidence interval (PL(x,ndf), Pu(x,ndf)) as follows:
PL(x,ndf) = viFvi,v2 (0.025)/(v2 + viFvi,V2(0.025))
Pu(X,ndf) = V3Fv3,v4 (0.975)/(V4 +V3Fv3,v4(0.975))
where x is equal to p times ndf, vi = 2x, V2 = 2(ndf - x + 1), V3 = 2(x + 1), V4 = 2(ndf - x), and Fdi,d2(P) is
the (3 quantile of an F distribution with dl and d2 degrees of freedom. (Note: If ndf is greater than the
actual sample size or if p is equal to zero, then the actual sample size should be used.) This step will
produce a lower and an upper limit for the estimated proportion obtained in Step 2.
Step 4: Use SAS Proc Univariate (again using the Weight option to assign weights) to determine the
chemical percentile values Pcdc, Lcdc and UcDcthat correspond to the proportion p obtained in Step 2 and
its lower and upper limits obtained in Step 3. Do not round the values of p and the lower and upper limits.
For example, if p = 0.4832, then Pcdc is the 48.32'th percentile value of the chemical. The alternative
percentile estimates Pcdc and Psas are not necessarily equal.
Step 5: Use the confidence interval from Step 4 to estimate the standard error of the estimated percentile
Pcdc:
Standard Error (Pcdc) = (Ucdc - Lcdc) / (2tdenom)
Step 6: Use the estimated percentile Pcdc and the standard error from Step 4 to estimate the relative
standard error of the estimated percentile Pcdc:
Relative Standard Error (%) = [Standard Error (Pcdc) / Pcdc] x 100%
The tabulated estimated percentile is the value of Psas given in Step 1. The relative standard error is given
in Step 6, using Pcdc and its standard error.
The relative standard error depends upon the survey design. For this purpose, the public release
version of NHANES includes the variables SDMVSTRA and SDMVPSU, which are the Masked
Variance Unit pseudo-stratum and pseudo-primary sampling unit (pseudo-PSU). For
approximate variance estimation, the survey design can be approximated as being a stratified
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random sample with replacement of the pseudo-PSUs from each pseudo-stratum; the true stratum
and PSU variables are not provided in the public release version to protect confidentiality. If the
relative standard error is too high, then the estimated percentile will not be accurately estimated.
Furthermore, if the degrees of freedom (from Step 2) is too low, then the relative standard error
will be less accurately estimated and thus may be underestimated. For these reasons, percentiles
with high relative standard errors or with low degrees of freedom are unstable or unreliable.
Percentiles with a relative standard error less than 30% and with 12 or more degrees of freedom
were treated as being reliable and were tabulated. Percentiles with a relative standard error that is
30% or greater but less than 40% and with 12 or more degrees of freedom were treated as being
unstable; these values were tabulated but were flagged to be interpreted with caution. Percentiles
with a relative standard error less than 40% and with between 7 and 11 degrees of freedom were
also treated as being unstable; these values were tabulated but were flagged to be interpreted
with caution. Percentiles with a relative standard error that is 40% or greater, or without an
estimated relative standard error, or with 6 or less degrees of freedom, were treated as being
unreliable; these values were not tabulated and were flagged as having a large uncertainty.
Questions and Comments
Questions regarding these methods, and suggestions to improve the description of the methods,
are welcome. Please use the "Contact Us" link at the bottom of any page in the America's
Children and the Environment website.
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Statistical Comparisons
Statistical analyses of the percentiles were used to determine whether the differences between
percentiles for different demographic groups were statistically significant. For these analyses, the
percentiles and their standard errors were calculated for each combination of age group, sex (in
the cases of children), income group (below poverty, at or above poverty, unknown income), and
race/ethnicity group using the method described in the "Relative Standard Error" section. In the
notation of that section, the percentile and standard error are the values of Pcdc and Standard
Error (Pcdc), respectively. These calculated standard errors account for the survey weighting and
design and, for women, for the age-specific birth rate.
Using a weighted linear regression model, the percentile was assumed to be the sum of
explanatory terms for age, sex, income and/or race/ethnicity and a random error term; the error
terms were assumed to be approximately independent and normally distributed with a mean of
zero and a variance equal to the square of the standard error. In this model, the weight is the
inverse of the variance, so that percentiles with larger standard errors are given less of a
statistical weight in the fitted regression model. Using this model, the difference in the value of a
percentile between different demographic groups is statistically significant if the difference
between the corresponding sums of explanatory terms is statistically significantly different from
zero. A p-value at or below 0.05 implies that the difference is statistically significant at the 5%
significance level. No adjustment is made for multiple comparisons.
For each type of comparison, we present unadjusted and adjusted analyses. The unadjusted
analyses directly compare a percentile between different demographic groups. The adjusted
analyses add other demographic explanatory variables to the statistical model and use the
statistical model to account for the possible confounding effects of these other demographic
variables. For example, the unadjusted race/ethnicity comparisons use and compare the
percentiles between different race/ethnicity pairs. The adjusted race/ethnicity comparisons use
the percentiles for each age/sex/income/race/ethnicity combination. The adjusted analyses add
age, sex, and income terms to the statistical model and compare the percentiles between different
race/ethnicity pairs after accounting for the effects of the other demographic variables. For
example, if White non-Hispanics tend to have higher family incomes than Black non-Hispanics,
and if the urine perchlorate level strongly depends on family income only, then the unadjusted
differences between these two race/ethnicity groups would be significant but the adjusted
difference (taking into account income) would not be significant.
Comparisons between pairs of race/ethnicity groups are shown in Table 1 for women ages 16 to
49 years and in Table 4 for children ages 6 to 17 years. Comparisons between income groups are
shown in Table 2 for women ages 16 to 49 years and in Table 5 for children ages 6 to 17 years.
In Tables 1 and 4, for the unadjusted "All incomes" comparisons, the only explanatory variables
are terms for each race/ethnicity group. For these unadjusted comparisons, the statistical tests
compare the percentiles for each pair of race/ethnicity groups. For the adjusted "All incomes
(adjusted for age, sex, income)" comparisons, the explanatory variables are terms for each
race/ethnicity group together with terms for each age, sex (for children), and income group. For
these adjusted comparisons, the statistical test compares the pair of race/ethnicity groups after
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accounting for any differences in the age, sex (for children) and income distributions between the
race/ethnicity groups. The adjustment for sex is applicable only for children, and thus appears
only in Tables 4, 5 and 6.
In Tables 1 and 4, for the unadjusted "Below Poverty Level" and "At or Above Poverty Level"
comparisons, the only explanatory variables are terms for each of the twelve
race/ethnicity/income combinations (combinations of four race/ethnicity groups and three
income groups). For example, in row 1, the p-value for "Below Poverty Level" compares White
non-Hispanics below the poverty level with Black non-Hispanics below the poverty level. The
same set of explanatory variables are used in Tables 2 and 5 for the unadjusted comparisons
between one race/ethnicity group below the poverty level and the same race/ethnicity group at or
above the poverty level. The corresponding adjusted analyses include extra explanatory variables
for age and sex (for children), so that race/ethnicity/income groups are compared after
accounting for any differences due to age or sex. Although these comparisons only involve the
two income groups with known incomes, these statistical models were fitted to all three income
groups (including those with unknown income) to make a more general, better fitting model; this
approach has no impact on the unadjusted p-values but has a small impact on the adjusted p-
values. Also in Tables 2 and 5, the unadjusted p-value for the population "All" compares the
percentiles for women ages 16 to 49 years or children ages 6 to 17 years below poverty level
with those at or above poverty level, using the explanatory variables for the two income groups
(below poverty, at or above poverty), excluding those with unknown income. The adjusted p-
value includes adjustment terms for age, sex (for children), and race/ethnicity in the model.
Additional comparisons are shown in Table 3 for women ages 16 to 49 years and in Table 6 for
children ages 6 to 17 years. Comparisons are shown for differences between children's age
groups, between those below poverty and those at or above poverty, and for changes over time
(trends). The Against = "age" unadjusted p-value compares the percentiles between children in
different age groups, using the explanatory variables for the age groups. The adjusted p-value
includes adjustment terms for sex (for children), race/ethnicity, and income in the model. The
Against = "income" unadjusted p-value compares the percentiles for those below poverty level
with those at or above poverty level, using the explanatory variables for the two income groups
(below poverty, at or above poverty). The adjusted p-value includes adjustment terms for age,
sex (for children), and race/ethnicity in the model. The Against = "year" p-value examines
whether the linear trend in the percentiles is statistically significant (using the percentiles for
each NHANES period regressed against the midpoint of that period); the adjusted model for
trend adjusts for demographic changes in the populations from year to year by including terms
for age, sex (for children), income, and race/ethnicity. The adjustment for sex is applicable only
for children, and thus appears only in Table 6.
For women, the age groups used were 16-19, 20-24, 25-29, 30-39, and 40-49. For children, the
age groups used were 6-10, 11-15, and 16-17.
For more details on these statistical analyses, see the memorandum by Cohen (2010).V1
vl Cohen, J. 2010. Selected statistical methods for testing for trends and comparing years or demographic groups in
ACE NHIS and NHANES indicators. Memorandum submitted to Dan Axclrad. EPA. 21 March. 2010.
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Table 1. Statistical significance tests comparing the percentiles of perchlorate in women ages 16
to 49 years, between pairs of race/ethnicity groups, for 2011-2014.
P-VALUES
Variable
Percentile
First
race/ethnicity
group
Second
race/ethnicity
group*
All
incomes
All
incomes
(adjusted
for age,
income)
Below
Poverty
Level
Below
Poverty
Level
(adjusted
for age)
At or
Above
Poverty
Level
At or
Above
Poverty
Level
(adjusted
for age)
Perchlorate
50
White non-
Hispanic
Black non-
Hispanic
0.750
0.635
0.268
0.184
0.967
0.563
Perchlorate
50
White non-
Hispanic
Mexican-
American
0.045
<0.001
0.273
<0.001
0.019
0.263
Perchlorate
50
White non-
Hispanic
Other
0.412
0.012
0.169
0.571
0.784
0.930
Perchlorate
50
Black non-
Hispanic
Mexican-
American
0.100
<0.001
0.989
0.006
0.021
0.208
Perchlorate
50
Black non-
Hispanic
Other
0.263
0.202
0.034
0.126
0.823
0.560
Perchlorate
50
Mexican-
American
Other
0.004
<0.001
0.034
<0.001
0.009
0.279
Perchlorate
95
White non-
Hispanic
Black non-
Hispanic
0.171
0.655
0.491
<0.001
0.051
0.179
Perchlorate
95
White non-
Hispanic
Mexican-
American
0.274
<0.001
0.919
0.414
0.037
<0.001
Perchlorate
95
White non-
Hispanic
Other
0.069
0.021
0.810
0.626
0.129
0.030
Perchlorate
95
Black non-
Hispanic
Mexican-
American
0.973
<0.001
0.463
<0.001
0.284
<0.001
Perchlorate
95
Black non-
Hispanic
Other
0.923
0.067
0.618
<0.001
0.725
0.184
Perchlorate
95
Mexican-
American
Other
0.909
<0.001
0.751
0.651
0.201
<0.001
* "Other" represents the "All Other Races/Ethnicities" category, which includes all other races and ethnicities not specified, together with those
individuals who report more than one race.
Table 2. Statistical significance tests comparing the percentiles of perchlorate in women ages 16
to 49 years, between those below poverty level and those at or above poverty level, for 2011-
2014.
P-Values for difference between income levels
V ariable
Percentile
Population*
Unadjusted
Adjusted (for age)**
Perchlorate
50
All
0.390
< 0.001
Perchlorate
50
White non-Hispanic
0.383
0.001
Perchlorate
50
Black non-Hispanic
0.658
0.833
Perchlorate
50
Mexican-American
0.132
0.827
Perchlorate
50
Other
0.065
0.008
Perchlorate
95
All
0.939
< 0.001
Perchlorate
95
White non-Hispanic
0.906
0.009
Perchlorate
95
Black non-Hispanic
0.730
0.715
Perchlorate
95
Mexican-American
0.044
< 0.001
Perchlorate
95
Other
0.367
< 0.001
* "Other" represents the "All Other Races/Ethnicities" category, which includes all other races and ethnicities not specified, together with those
individuals who report more than one race.
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** Comparison for "All" is adjusted for age and race/ethnicity; comparisons for race/ethnicity categories are adjusted for age.
Table 3. Other statistical significance tests comparing the percentiles of perchlorate in women
ages 16 to 49 years, for 2011-2014 (trends for 2001-2014).
P-VALUES
Variable
Percentile
From
To
Against
Unadjusted
Adjusted*
Perchlorate
50
2011
2014
income
0.390
< 0.001
Perchlorate
50
2001
2014
year
0.001
< 0.001
Perchlorate
95
2011
2014
income
0.939
< 0.001
Perchlorate
95
2001
2014
year
0.001
< 0.001
*For Against = "income," the comparison is between those below the poverty level and those at or above the poverty level, and the p-values are
adjusted for age and race/ethnicity.
For Against = "year" the comparison is the trend over different years, and the p-values are adjusted for age, race/ethnicity, and income.
Table 4. Statistical significance tests comparing the percentiles of perchlorate in children ages 6
to 17 years, between pairs of race/ethnicity groups, for 2011-2014.
P-VALUES
At or
All
Below
Above
First
Second
incomes
(adjusted
Below
Poverty
Level
At or
Above
Poverty
Level
Variable
Percentile
race/ethnicity
group
race/ethnicity
group*
All
incomes
for age,
income)
Poverty
Level
(adjusted
for age)
Poverty
Level
(adjusted
for age)
Perchlorate
50
White non-
Hispanic
Black non-
Hispanic
0.772
0.456
0.214
0.908
0.580
0.937
Perchlorate
50
White non-
Hispanic
Mexican-
American
0.778
0.781
0.951
0.314
0.814
0.415
Perchlorate
50
White non-
Hispanic
Other
0.203
0.206
0.523
0.569
0.123
0.049
Perchlorate
50
Black non-
Hispanic
Mexican-
American
1.000
0.613
0.322
0.433
0.557
0.305
Perchlorate
50
Black non-
Hispanic
Other
0.369
0.014
0.469
0.561
0.399
0.007
Perchlorate
50
Mexican-
American
Other
0.381
0.076
0.647
0.130
0.206
<0.001
Perchlorate
95
White non-
Hispanic
Black non-
Hispanic
0.687
<0.001
0.316
0.015
0.837
0.533
Perchlorate
95
White non-
Hispanic
Mexican-
American
0.589
<0.001
0.429
<0.001
0.654
0.113
Perchlorate
95
White non-
Hispanic
Other
0.803
0.192
0.863
0.009
0.816
0.139
Perchlorate
95
Black non-
Hispanic
Mexican-
American
0.745
0.129
0.604
0.004
0.615
<0.001
Perchlorate
95
Black non-
Hispanic
Other
0.391
<0.001
0.141
0.871
0.722
<0.001
Perchlorate
95
Mexican-
American
Other
0.471
<0.001
0.069
<0.001
0.747
0.529
* "Other" represents the "All Other Races/Ethnicities" category, which includes all other races and ethnicities not specified, together with those
individuals who report more than one race.
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Table 5. Statistical significance tests comparing the percentiles of perchlorate in children ages 6
to 17 years, between those below poverty level and those at or above poverty level, for 2011-
2014.
P-Values for difference between income levels
V ariable
Percentile
Population*
Unadjusted
Adjusted (for age, sex)**
Perchlorate
50
All
0.125
0.565
Perchlorate
50
White non-Hispanic
0.072
0.693
Perchlorate
50
Black non-Hispanic
0.817
0.572
Perchlorate
50
Mexican-American
0.215
0.040
Perchlorate
50
Other
0.763
0.006
Perchlorate
95
All
0.635
0.001
Perchlorate
95
White non-Hispanic
0.549
0.090
Perchlorate
95
Black non-Hispanic
0.366
0.019
Perchlorate
95
Mexican-American
0.731
< 0.001
Perchlorate
95
Other
0.398
< 0.001
* "Other" represents the "All Other Races/Ethnicities" category, which includes all other races and ethnicities not specified, together with those
individuals who report more than one race.
** Comparison for "All" is adjusted for age, sex, and race/ethnicity; comparisons for race/ethnicity categories are adjusted for age and sex.
Table 6. Other statistical significance tests comparing the percentiles of perchlorate in children
ages 6 to 17 years, for 2011 -2014 (trends for 2001-2014).
P-VALUES
Variable
Percentile
From
To
Against
Unadjusted
Adjusted*
Perchlorate
50
2011
2014
age
0.025
< 0.001
Perchlorate
50
2011
2014
income
0.125
0.565
Perchlorate
50
2001
2014
year
<0.001
< 0.001
Perchlorate
95
2011
2014
age
0.211
< 0.001
Perchlorate
95
2011
2014
income
0.635
0.001
Perchlorate
95
2001
2014
year
0.112
< 0.001
*For Against = "age," the comparison is between the age groups 6-10, 11-15, and 16-17 years, and the p-values are adjusted for sex,
race/ethnicity and income.
For Against = "income," the comparison is between those below the poverty level and those at or above the poverty level, and the p-values are
adjusted for age, sex, and race/ethnicity.
For Against = "year" the comparison is the trend over different years, and the p-values are adjusted for age, sex, race/ethnicity, and income.
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