Perspective: comparing two-sample tests and control charts.

For the task of monitoring a process, detecting change, and making correct attributions on a continuous basis, there is no better tool than the control chart, particularly where the data set is large and permits subgrouping. By using this technique, staff will get quicker answers or will have a smaller data requirement, and will arrive at more valid conclusions than with the two-sample method. (The two-sample approach is typically used when the data are not produced or collected in sequence is unknown. This, however, may reflect a weakness in the data collection process.) In addition, we believe that the control chart approach is as simple as the two-sample approach. Once the control limits and zones are established, the plot of points can be maintained by personnel who need to do no more than plot a rate on a weekly (or a daily) basis. The control chart rules properly focus laboratory and QA personnel on change. Control chart procedures are statistical tests of special cause and need no further significance tests. Those using control charts need to be aware of three important points: 1. Control limits are not standards or thresholds and cannot be derived from these concepts. Control limits are a statement about the current natural variation in the process. 2. The type of chart to be used should depend upon the type of data being evaluated. (Finison et al., 1993).(ABSTRACT TRUNCATED AT 250 WORDS)