A steel manufacturer makes large I-beams for the construction industry. I-beams are large, expensive to manufacture and are made infrequently. One of the critical features, the "A-Dimension," must be closely controlled and is difficult to measure.

If the A-Dimension average sustains a shift of more than 1? above its overall target, then the strength of the product could be compromised and product could be scrapped, resulting in significant financial loss. To prevent these issues, the company wants line operators to be notified when a shift of at least one ? has occurred in the A-Dimension's average.

Data has been collected for the last 28 A-Dimension values. They are found in the table below. Assume that the first 20 points are for sure in-control.

Subgroup # A-Dimension 50.453 2 50.682 3 49.686 4 49.572 5 51.333 6 50.280 7 49.240 8 50.478 9 49.263 10 50.046 11 49.540 12 49.270 13 50.316 14 49.512 15 49.895 16 50.014 17 49.373 18 50.523 19 51.111 20 50.044 21* 51.601 22* 50.479 23* 49.089 24* 50.632 25* 50.373 26* 51.682 27* 50.521 28* 51.639