5 In a collaborative trial, two closely similar samples of oil shale (A and B) were sent to 15 laboratories, each of which performed a single inductively coupled plasma determination of the cadmium level in each sample. The following results were obtained: Plot the two-sample chart for these data, and comment on the principal source of error in the collaborative trial. Estimate the overall variance, the measurement variance, and the systematic error component of the variance of the results