This example is taken from an interesting article by Joiner (1981) and from data in Ryan...

This example is taken from an interesting article by Joiner (1981) and from data in Ryan, Joiner, and Ryan (1976). The National Institute of Standards and Technology supplies standard materials of many varieties to manufacturers and other parties, who use these materials to calibrate their own testing equipment. Great pains are taken to make these reference materials as homogeneous as possible. In an experiment, a long homogeneous steel rod was cut into 4-inch lengths, 20 of which were randomly selected and tested for oxygen content. Two measurements were made on each piece. The 40 measurements were made over a period of 5 days, with eight measurements per day. In order to avoid possible bias from time-related trends, the sequence of measurements was randomized. The file steelrods contains the measurements. There is an unexpected systematic source of variability in these data. Can you find it by making an appropriate plot?Would this effect have been detectable if the measurements had not been randomized over time?