A potential criticism of analyzing these data as if they were two independent samples is that the measurements taken in 1996 were taken at the same sites as the measurements taken in 1982. Thus, there is the possibility that there will be a strong positive correlation between the pair of observations at each site.
a. Plot the pairs of observations in a scatterplot with the 1982 values on the horizontal axis and the 1996 values on the vertical axis. Does there appear to be a positive correlation between the pairs of measurements? Estimate the correlation between the pairs of observations?
b. Compute the correlation coefficient between the pairs of observations. Does this value confirm your observations from the scatterplot? Explain your answer.
c. Answer the questions posed in parts (a) and (b) of Exercise 6.11 using a paired data analysis. Are your conclusions different from the conclusions you reached treating the data as two independent samples?