Percent Deviation

The percent deviation formula is very useful in determining how accurate the data collected by student teams is. In this case the data is usually compared either to a standard prepared by the instructor or by a quality control lab (like the City of Gresham or United Sewage Agency).

Note that a team can be very precise in their measurements and have a high coefficient of variance for a set of data (say 96%) which is highly desirable, and still they may not have accurate test results. This means that they repeatedly made the same mistake in collecting data. The team would have to create a graph comparing the sampled data to the quality control data or they could calculate percent deviation using the following equation:

Percent deviation = (student data - lab data) / lab data then multiplied by 100.

or

Percent deviation = (student data - QC data) / QC data then multiplied by 100.

Note:

a. If the percent deviation is a negative number, that means the student data is lower than the lab determined value.

b. If the percent deviation is a positive number, that means the student data is higher than the lab determined value.

Example: If our measured average nitrate reading was 1.4 mg/L N and the lab QC reading was 1.2 mg/L N, the

[Click here for Quality Control Comparisons of Student Data]

Courtesy of the Student Watershed Research Project.

Slichter