United States
Environmental Protection
\r ^1 # ^Agency
Office of Water
EPA 82 l-F-21-005
November 2021
Fact Sheet: Alternative Methods Calculator Tool
Summary
The Alternative Methods Calculator Tool (AltCalc
Tool) is an Excel-based tool that can be used to
determine whether an alternative microbial water
quality method can be used to replace an EPA
approved method when deriving site-specific
alternative recreational water quality criteria
(RWQC). The AltCalc Tool should be used in
conjunction with EPA's Site-Specific Alternative
Recreational Criteria Technical Support Materials for
Alternative Indicators and Methods (Alternative
Methods TSM; 2014, EPA-820-R-14-011).
Specifically, the AltCalc Tool is used to calculate the
index of agreement (IA) and Pearson's correlation
coefficient squared (R-squared) for microbial water
quality datasets from two different methods; these
values are used to assess whether the alternative
method is appropriate for replacing using an existing
method.
Background
EPA's 2012 Recreational Water Quality Criteria (820-
F-12-058) includes a discussion of tools for
developing alternative criteria, including utilization
of alternative indicators and methods. EPA's
Alternative Methods TSM helps state and local
authorities who are developing site-specific
alternative criteria using new methods for fecal
indicator detection or enumeration that EPA has not
validated and issued. New methods or indicators
might provide improvements over existing methods
with regards to speed, sensitivity, specificity, lower
cost, ease of use, and method performance for
utilization of microbial indicators in site-specific
alternative criteria to replace EPA's recommended
national RWQC methods. The TSM describes how an
EPA-approved method (referred to as method 1) can
be compared to an alternative method (referred to
as method 2) in order to determine if method 2 can
be used to replace method 1. Method 1 includes any
of the following:
¦	EPA Method 1600 for enterococci
¦	EPA Method 1603 for Escherichia coli (E. coli)
¦	EPA Method 1611 for Enterococcus spp.
¦	Any "equivalent" method to those listed above,
as determined by the Alternate Test Procedure
(ATP) program
¦	Future methods that EPA recommends for CWA
§304(a) ambient water quality criteria
The Alternative Methods TSM outlines the scientific
information that is needed before an alternative
indicator/method, or Method 2, can replace the use
of an EPA recommended or approved method on a
site-specific basis. The third step in the Alternative
Methods TSM requires users to compare the two
indicator/methods (Method 1 and 2) using the
statistical calculation of IA and R-squared for paired
datasets. Appendix E in the Alternative Methods
TSM outlines how to perform these calculations.
After data are entered into the AltCalc Tool, these
calculations are performed automatically based on
equations built into the tool.
What data do users provide?
Users provide microbial water quality data from the
site for which site-specific alternative criteria are to
be derived. For each water sample, two methods are
tested, an EPA approved method and the new
alternative method. At least 30 paired data points
that are within the limits of quantification are
needed to perform the comparison in the AltCalc
Tool.

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What calculations does the AltCalc Tool
perform?
Water quality data are input into the AltCalc Tool
which then performs the statistical comparison
between the resulting data from the two water
quality methods in order to determine whether an
alternative method can be used for site-specific
alternative criteria.
The AltCalc Tool calculates IA and R-squared:
¦	The IA describes the model fit between the
output and the input data. It can vary between 0
and 1, with 1 being a perfect fit. If IA is > to 0.7,
the site-specific criteria values for the new
indicator/method can be the same numerical
criteria values as for the EPA indicator/method. If
IA does not indicate good agreement (i.e., IA <
0.7), TSM users can use the R-squared between
the two methods to determine whether the
indicators are correlated.
¦	R-squared is a statistical measure of how close
the data are to the fitted regression line. It can
vary between 0 and 1, with 1 being perfect
correlation. If the alternative indicator/method
correlates with the EPA method, based on the R-
squared value (R-squared > 0.6), users can derive
new criteria values using the process described
in the Alternative Methods TSM.
The resulting information is used to determine
whether the two methods produce a consistent and
predictable relationship. If so, then users may
replace the EPA-approved water quality method
with an alternative method.
Where can I find more information?
Access the AltCalc Tool at:
https://www.epa.gov/svstem/files/other-files/2021-
11/site-specific-alt-calculator-tool.xlsm
View EPA's Alternative Methods TSM at:
https://www.epa.gov/sites/default/files/2015-
11/documents/sitespecific-alternative-recreational-
indicators-methods.pdf
For technical support using the AltCalc Tool, email
Shamima Akhter at: akhter.shamima@epa.gov

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