10.48 Predicting water quality. The index of biotic integrity (IBI) is a measure of the water quality in streams. IBI and land use measures for a collection of streams in the Ozark Highland ecoregion of Arkansas were collected as part of a study.21 Table 10.4 gives the data for IBI, the percent of the watershed that was forest, and the area of the watershed in square kilometers for streams in the original sample with watershed area less than or equal to 70 km2.
(a) Use numerical and graphical methods to describe the variable IBI. Do the same for area. Summarize your results.
TABLE 10.4: Watershed Area (km2), Percent Forest, and Index of Biotic IntegrityArea Forest IBI Area Forest IBI Area Forest IBI Area Forest IBI Area Forest IBI 21 0 47 29 0 61 31 0 39 32 0 59 34 0 72 34 0 76 49 3 85 52 3 89 2 7 74 70 8 89 6 9 33 28 10 46 21 10 32 59 11 80 69 14 80 47 17 78 8 17 53 8 18 43 58 21 88 54 22 84 10 25 62 57 31 55 18 32 29 19 33 29 39 33 54 49 33 78 9 39 71 5 41 55 14 43 58 9 43 71 23 47 33 31 49 59 18 49 81 16 52 71 21 52 75 32 59 64 10 63 41 26 68 82 9 75 60 54 79 84 12 79 83 21 80 82 27 86 82 23 89 86 26 90 79 16 95 67 26 95 56 26 100 85 28 100 91 (b) Plot the data and describe the relationship between IBI and area. Are there any outliers or unusual patterns?
(c) Give the statistical model for simple linear regression for this problem.
(d) State the null and alternative hypotheses for examining the relationship between IBI and area.
(e) Run the simple linear regression and summarize the results.
(f) Obtain the residuals and plot them versus area. Is there anything unusual in the plot?
(g) Do the residuals appear to be approximately Normal? Give reasons for your answer.
(h) Do the assumptions for the analysis of these data using the model you gave in part (c) appear to be reasonable? Explain your answer.