STA 6166 UNIT 2 Graded Assignment Answer Form
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Unit 2 Graded Assignment Answer Form

An Ecological Risk Assessment Example

Report of (First Name) (Last Name)

Instructions: Read this report and fill in all requested text boxes and radio buttons. Everything requested here should have been on the homework report you submitted for the Unit 2 Graded Assignment.


Mercury (Hg) and its compounds have no known metabolic function in biota but their presence in cells of living organisms represents contamination from natural or anthropogenic sources. Mercury is used in a number of industrial processes such as paper manufacturing and as an agricultural fungicide. As part of an ecological risk assessment of an old industrial site suspected of having soils contaminated with mercury, the study team decided to compare the average levels of mercury in the fur of raccoons (Procyon lotor) living on the study site to those living in a far removed area (background). The study objective was to determine if there is a difference between the two sites and in particular whether average concentration levels in fur from study site raccoons is elevated above average concentration levels of raccoon fur from the background site. If this is the case, we will conclude that there is increased risk of ecological damage from the study site.

Published data indicate that the average level of mercury in raccoon fur is about (m=)1.12 on a natural log scale (e.g. about 3.8 mg/kg concentration) with a standard deviation of about (s=)0.4 on a natural log scale( or 1.38 mg/kg concentration). Because the natural log of concentration data is typically more normally distributed than is untransformed concentration data, all computations and measurements are performed using the natural log transformed values. Ecologically important effects are expected if the average log concentration between the two sites differs by 0.56 (on the log scale) or roughly half again more mercury accumulation in fur in the study site than what is expected from the background site.We do not want to initiate cleanup and legal proceedings unless we are sure that any difference observed is real and greater than the prespecified ecologically important level. For this reason we set a fairly stringent Type I error rate of a=0.01 and a Type II error rate of b=0.05 for the computation of needed sample size.

Analysis of mercury in fur samples is difficult. To increase the certainty that the results coming back from or primary lab are the true (log) concentration values we performed a small interlaboratory comparison. We split three samples each site and send one of the two subsamples to a second lab for a more accurate and expensive analysis. Results of this comparison must be reported as well.


Statistical Analysis

Based on the information found in the literature review and summarized in the Introduction, the necessary sample size for the study was determined using the following equation obtained from Ott and Longnecker (2001).


where s is the suspected joint population variance, za and zb are tail percentiles from a standard normal distribution for the specified type I and type II error rates and D is the difference in concentration determined to be ecologically important in this setting.

The predetermied number of hair samples were collected from captured raccoons at both the study site and the background area. The hypothesis of equal murcury concentrations was tested using a two-independent sample t-test, furthur confirmed with a Mann-Whitney-Wilcoxon non-parameteric test for equal medians, both performed at the a = 0.05 Type I error (significance) level. The particular form of the two-independent sample t-test was chosen as a result of an F-test for equal group variances. Means, standard errors and test associated p-values are reported.

Hair samples from three raccoons from both sites (a total of 6 samples) were split into two subsamples with one subsample being processed by the same lab as all other samples in the study and the other subsample being sent to a second independent lab for confirmation. A paired samples t-test (with n=6) was used to test the null hypothesis of no lab differences in average concentration. The result of the test was further confirmed via the Wilcoxon signed ranks test of the differences.


Sample Size Determination

Using Equation 1, a sample of (enter number of samples to be collected from each area here, ie. total number of samples is 2xn, enter n here.) raccoon hair samples were collected from both the study site and the background area.

Site versus Background Comparisons

The F test for homogeneity of variance ( F statistic value= (enter your value here) and associate p-value= (enter p-value for test or <0.05 or >0.05 if performed by hand) suggested use of the (choose one) Pooled Variance 2 sample t-test, Separate variance 2 sample t-test. A one-tailed test was performed to examine the alternative hypothesis that Site concentrations were higher than Background concentrations. The results of this test ( t= (place your t-statistic value here) with df= (place degrees of freedom here)) suggested that average mercury concentrations in raccoon hair were (choose one) significantly different, not significantly different. The Mann-Whitney-Wilcoxon rank sum test (W= (place test statistic here)) (choose one) confirmed, did not confirm the two-sample t-test.

Basic statistics for the two study areas are presented in the table 1.

Table 1. Basic statistics on mercury concentration for study areas.

Sample Size
Standard Error


Lab Comparisons

Differences between the study lab and the confirmation lab was computed for each of the six pairs of concentrations values obtained in the study. These values were subjected to a one-sample t-test to determine whether labs differed in average concentration amounts. The test results (t= , p-value= ) suggested that average concentrations were statistically different not different (choose one). The one sample signed ranks test for zero median (W= (enter test statistic value), p-value= ) was, was not in agreement with the findings of the t-test.


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NOTE: This example is totally synthetic and the figment of Dr. Portier's vivid imagination. Any similarity to real cases or situations is completely incidental. No claim is made as to the risk from Mercury. No raccoons were harmed as part of this synthetic study.