27. An X and MR chart is to be maintained on the pH value for the swimming pool water of a leading motel.

One reading is taken each day for 30 days. Data are 7.8, 7.9, 7.7, 7.6, 7.4, 7.2, 6.9, 7.5, 7.8, 7.7, 7.5, 7.8, 8.0, 8.1, 8.0, 7.9, 8.2, 7.3, 7.8, 7.4, 7.2, 7.5, 6.8, 7.3, 7.4, 8.1, 7.6, 8.0, 7.4, and 7.0. Plot the data on graph paper, determine the trial central line and limits, and evaluate the variation.