Figure 2-2. Each dot identifies an EMAP-Virginian Province station location in estuaries, 1990-1993. 



Guideline 8: Estimation of Measurement Error 



The process of collecting, transporting, and analyzing ecological data generates errors that can obscure 

 the discriminatory ability of an indicator. Variability introduced by human and instrument performance 

 must be estimated and reported for all indicator measurements. Variability among field crews should 

 also be estimated, if appropriate. If standard methods and equipment are employed, information on 

 measurement error may be available in the literature. Regardless, this information should be derived 

 or validated in dedicated testing or a pilot study. 



Using the QA information collected by EMAP over the period fronn 1991 to 1993 (a different method was 

 employed in 1990, so those data were excluded from this analysis), we can estimate the error associated 

 with this measurement. Figure 2-3 is a frequency distribution for 784 stations of the absolute difference 

 between the DO measurements collected by the CTD and the DO meter used as a cross check (A DO). The 

 data included in this figure were collected over three years by nine different field crews. Therefore, the 

 figure illustrates the total measurement error-that associated with instrumentation as well as with operation 

 of the instruments. Of the 784 stations, the measurement quality objective of < 0.5 mg/L was met at over 90 

 percent. No bias was detected, meaning the CTD values were not consistently higher or lower than those 

 from the DO meter. 



It is of course possible to analyze instrumentation and operation errors separately. Such analyses would be 

 necessary if total error exceeded a program's measurement quality objectives, in order to isolate and attempt 



2-8 



