Question: The same item is being produced on two production lines. Every 15 min, 5 items are sampled from each line, and a feature of interest is measured on each item. Some statistic Q is calculated for each set of 5 measurements from each line and plotted versus time. Analyst 1 puts all 10 items together into a single group (5 from line 1 and 5 from line 2), calculates a value of the statistic Q, and plots it. (This person says, "After all, is not a larger sample size better?") Analyst 2 keeps the data from the two different lines separate and makes a different control chart for each production line.
(a) What subgroup size is Analyst 1 using?
(b) What subgroup size is Analyst 2 using? (c) Which analyst is making the most appropriate chart? Why?