In previous articles, we discussed the advantages that Xbar charts have over Individuals charts in detecting process shifts. (See “How Should the Subgroup Size be Selected for an Xbar Chart (Part I)”) We saw that charts of Individuals are ineffective at quickly detecting small process shifts (and detecting these small process changes may be critical!).
Charts of averages (Xbar) are superior because averaging the subgroup data gives us greater certainty as to where the process is actually running at a point in time. Selecting an appropriate sample size on an Xbar chart allows us to align the statistical performance of the control chart with the desired practical process changes we’d like to detect. That is, the sensitivity (ability to detect change) may be adjusted based on the sample size utilized. [Read more…]