Mixtures. Here is an artificial situation that illustrates an unusual control chart pattern. Invoices are processed and paid by two clerks, one very experienced and the other newly hired. The experienced clerk processes invoices quickly. The new hire must often refer to a handbook and is much slower. Both are quite consistent, so that their times vary little from invoice to invoice. It happens that each sample of invoices comes from one of the clerks, so that some samples are from one and some from the other clerk. Sketch the chart pattern that will result.