An article in Quality Engineering (Vol. 4, 1992, pp. 487-495) presents viscosity data from a batch chemical process. A sample of these data follows on p. 219:

(a) Reading down and left to right, draw a time series plot of all the data and comment on any features of the data that are revealed by this plot.

(b) Consider the notion that the first 40 observations were generated from a specific process, whereas the last 40 observations were generated from a different process. Does the plot indicate that the two processes generate similar results?

(c) Compute the sample mean and sample variance of the first 40 observations; then compute these values for the second 40 observations. Do these quantities indicate that both processes yield the same mean level? The same variabil- ity? Explain.