A chemical product was manufactured at two-week intervals, and after a period of storage the level of available chlorine, which is known to decline with time, was measured in cartons of the product (Draper and Smith, 1998, Chapter 24, Section 24.3, pp 518–519). The results obtained are presented in Table 1.9. (Data reproduced by permission of John Wiley & Sons, Inc.)

(a) Arrange these data for analysis by GenStat or R.

(b) Plot the values of available chlorine against the times since production. Obtain the line of best fit relating these two variables, treating each observation as independent, and display it on your plot.

(c) Consider whether observations made at the same time form a natural group. If so, consider whether it is reasonable to consider the variation of the group means around a fitted line as a random variable. These data will be analysed further in Chapter 7, Exercise 1.