Evaluation of Volunteer Data Quality

Last issue, we mentioned how some folks question the quality of data collected through volunteer programs (Winter 2006 issue of The Water Line). At that time LMVP staff were evaluating the reliability of LMVP data, with plans of presenting the findings at the National Monitoring Conference. The analyses are done, graphs are made, the slide show presented, and results indicate that data generated by the LMVP are reliable. What follows is a short review of the methods used to evaluate LMVP data and the results.

Comparison of Annual Averages
We compared data collected through the LMVP to data generated by other University of Missouri water quality monitoring projects. To qualify for this analysis a site had to be sampled at least three times during a summer by both the LMVP and MU. The result was a total of 178 comparisons (each comparison was an individual site during an individual year) representing 41 different sites from 29 lakes. Each comparison was statistically analyzed to determine if the LMVP average for a given site/year differed from the MU average.

Statistical analysis indicated that 164 (92%) of the total phosphorus comparisons were not significantly different. Results for the other parameters were even better and are presented in Table 1 below. Figure 1 shows the annual average total phosphorus data plotted out. The horizontal and vertical axes represent the MU and LMVP values, respectively. Each symbol denotes one of the 178 comparisons, and the dashed line is the 1:1 line (if LMVP and MU values were exactly the same the symbol would fall on the line). As you can see, most of the symbols are located near the 1:1 line indicating close agreement between LMVP and MU data. Most of the comparisons that fall farther away from the 1:1 line were sites with higher phosphorus values (>50 ug/L). This is to be expected because lakes that have higher nutrient levels also have higher variability.

Figure 1

Should we be concerned with the comparisons that were not in agreement? Given that Missouri lakes tend to be quite variable, differences in the timing of sample collection by LMVP and MU accounts for some of the differences in average values. Also, on some of the large reservoirs ( Lake of the Ozarks, Table Rock, etc.) the location of LMVP and MU sites were not perfectly matched. We feel that given the gradients in water quality found in large reservoirs, even a few miles difference between sample sites can lead to differences in average values.

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