HW2.Solutions - Biological Statistics II Biometry 3020 /...

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Biological Statistics II Biometry 3020 / Natural Resources 4130 / Stat Sci 3200 Homework 2 Due on Thursday February 11 1. Now that you are familiar with the grade point average data consider the diagnostics problem KNNL 3.3, do all components of this problem except for part e that uses the Brown-Forsythe test, which we did not cover in class. Note use a stem-and-leaf plot in place of doing a dot plot. a. (2 points) Prepare a box plot for the ACT scores Xi. Are there any noteworthy features in this plot? boxplot(CH01PR19$ACT) The boxplot looks pretty “normal”. It seems a symmetric, with most of the data clustered around the middle (kind of bell shaped) but with no extreme outliers. Appears to be what would come from a random sample. 15 20 25 30 35 b. (2 points) Prepare a stem-and-leaf plot of the residuals. What information does this plot provide? It looks like there are one or two outliers. We may be concerned if they act as leverage points. But, they may also influence our estimates of the variance and affect normality assumptions. stem(resid(GPA.lm)) N = 120 Median = 0.0406183 Quartiles = -0.3456104, 0.444013 Decimal point is at the colon
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Low: -2.740036 -1 : 8 -1 : 2221000 -0 : 99887766655555 -0 : 4444444333333322222211111111000000 0 : 000011111111222222233333344444444 0 : 5555555556666677777888888 1 : 00002 Here is what the stem() plot looks like in R (a little different): > CH01PR19 = read.table("CH01PR19.txt") > names(CH01PR19)=c("GPA","ACT") > GPA.lm = lm(GPA~ACT,data=CH01PR19) > stem(resid(GPA.lm)) The decimal point is 1 digit(s) to the left of the | -26 | 4 -24 | -22 | -20 | -18 | 3 -16 | -14 | -12 | 43 -10 | 7341 -8 | 887
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HW2.Solutions - Biological Statistics II Biometry 3020 /...

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