United States Office of Water EPA 821-F-01-001

Environmental Protection 4303 January 2001

Agency

Fact Sheet

Water Pollution and Water Supply Study

Performance Data 

Summary

Performance data from Water Pollution (WP) and Water Supply (WS) studies conducted by EPA from

1993 to 1999 are now available on the EPA Office of Science and Technology web page. EPA started the

WP and WS studies in the late 1970s to support laboratory accreditation programs. EPA maintained WP

and WS study data in a database and have used these data to evaluate performance of chemical methods

used in compliance monitoring programs. To develop method performance data, EPA developed regression

equations that characterize the interlaboratory precision and accuracy of hundreds of chemical methods

over a range of concentrations.

Background

EPA initiated WP and WS studies in the late

1970's. EPA sponsored the studies through 1999,

andnowsupportsprivatesectoroperation of this

program. We undertook the WP and WS studies

mainly to support state and other laboratory

accreditationprograms. Thanks to the participation

of more than 4,900 laboratories, the WP and WS

studiesproducedalargedatabaseof information

that demonstrates theperformanceof chemical

methods used in compliance monitoring programs

perform across the analytical community. The

performancedatapostedtoday comesfromWP

SeriesWP031toWP040,and WS Series WS032

to WS041.

How the Studies Worked : 1993-1999

Twice a year, we sent participant laboratories

“performance evaluation” (PE) samples. PE

sampleswereconcentrates containing high purity

chemical compounds (analytes) of interest. The

PE samples were provided as “single blind”

samples. In other words, laboratories knew that

thesamples were PEsamples, but didnot know

theconcentrations of the analytes in the samples.

Upon receiving a PE sample, the laboratory

selected an appropriatemethod,dilutedthe PE

sample per EPA specifications, analyzed the

sample, and reported the measured concentration

to EPA. 

EPAscientists comparedthe reported values to

the true concentrations of analytes in the PE

samples to determine how closely the

laboratory was in determining the correct

concentration.The accuracyofanalyte recovery

wasused in accreditation programs to determine

whether the laboratory performed at an acceptable

level.

Method Performance and Regression

Equations

The values reported from WP and WS studies

provided an enormous database of interlaboratory

data for hundreds of chemical methods. EPA used

thesedatatodevelopinterlaboratory precision and

accuracy statements thatindicatehowmethods

perform across the analytical community. 

EPAchose to usedatacollected between 1993

and 1999 because these data were the best

available.EPAscreenedfor“outliers” to remove

datathatwerebiasedduetoanalyst error or other

factorsunrelatedto the method. This screening

step ensured that the remaining data represented

the actual interlaboratory performance of the

method.

Usingavailabledata,EPAwasable to determine

howthe precision andaccuracyof the methods

varied as a factor of concentration. We used

regressionequations tosummarize this variability.

You can use these equations to predict recovery

and precision in reagent water based on

concentration, provided that the concentration falls

within the data used to generate the equation.
Analytes and Methods

WP and WS data exist for many analytes

including:

1. Trace metals

2. Nutrients

3. Insecticides/pesticides/herbicides

4. Carbamates

5. Polychlorinated bipheynls (PCBs)

6. Polyaromatic hydrocarbons (PAH’s)

7. Volatile organic compounds (VOCs)

8. Disinfection by-products

9. Minerals

10. Nutrients

The technologies used include:

11. Graphite Furnace Atomic Absorption

Spectroscopy (GFAAS)

12. Inductively Coupled Plasma - Mass

Spectrometry (ICP-MS)

13. Gas Chromatography (GC) 

14. Ion Chromatography

15. Spectrophotometry

Methods sources include:

16. EPA

17. ASTM 

18. Hach Company

19. Standard Methods

20. U.S. Geological Survey

What is Happening Now

SinceOctober24,1999,all WPand WSstudies

except for whole effluent toxicity testing have

performed by private companies . The National

Institute of Standards and Technology (NIST)

certifiesnon-EPAproficiency testing providers to

prepare PE samples and evaluate laboratory

performance. EPAcontinues to issue standard

operating procedures used in the WS and WP

program. 

Additional Information and Copies

Methodperformance data from the 1993-1999 WP

andWSstudiesare availablethrough the Internet

at http://www.epa.gov/ost/methods. 

For further information concerningWSand WP

studies, please contact the Analytical Methods

Staff in EPA's Office of Water. They can be

reachedby telephoneat 202-260-7120 or at the

following mailing address: 

U.S. Environmental Protection Agency

Office of Science and Technology

Analytical Methods Staff (4303)

1200 Pennsylvania Avenue, NW

Ariel Rios Building

Washington, DC 20460

A complete explanation of the data in the WS and

WP performance evaluation tables and the

statistical procedures used to develop the related

regressions will be available in the following

articles:

Britton,P.W., “Method-Specific Precision and Bias

Relationships Developed fromData Submitted

During USEPA Wastewater Laboratory

Performance Studies”. Journal of Testing and

Evaluation, JTEVA, Volume 28, Number 6,

November 2000, pp. 526-245. 

and

Britton, P.W., “Method-Specific Precision and Bias

Relationships Developed from Data Submitted

During USEPA Drinking Water Laboratory

Performance Studies,” Journal of Testing and

Evaluation, JTEVA, Volume 29, Number 1, January

2001, pp.79-112.