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| Contents | Index Sample Size When a file contains Attributes data, the Analysis Options dialog box provides for entry of the sample size. Choose one of the following three methods for specifying sample size: Note: The sample size must be greater than the largest count from the data you are analyzing for p and Np charts. Constant (p, u, np or c charts) Enter the sample size (the number of units inspected) if each of the count data you are analyzing was drawn from the same size sample. From Col (p or u charts) Enter the column containing the sample size (the number of units inspected) if each of the count data you are analyzing was drawn from varying size samples, and the sample size is contained in a column in the data file. The control limits will vary with the sample size: the true process average may be estimated with greater certainty with larger samples, resulting in tighter control limits. Base limits on mean sample size Check this to use the average of the sample sizes in the specified column to determine control limits. In this case, control limits will be constant for the region. Note: Using the average sample size is only recommended if the subgroup size does not vary by more than about 20%. If there is excessive variation in subgroup size, control chart sensitivity will be lost. Proportions entered (p, u, np or c charts) Enter the sample size (the number of units inspected) if each of the data you are analyzing is a proportion, and was drawn from the same size sample. |
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