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Using SAS Choose Design ANOVA Compare Means Regression Examples
ANOVA Diagnostics Start     5 of 8

C. Check normality (continued).

Three more examples of normality diagnostics are shown here to show the range of results and the conclusions they lead to.

Example 1. This illustrates excellent normality, with a W well above 0.90 (ignore the statistically significant p-value), and data following the reference normal distributions in the graphs.

Skewed Stem Leaf


Example 2. This example shows non-normality due to positive skew. A log transformation is almost certainly needed here (see Step E).

Peaked Stem Leaf

Example 2. This example shows poor normality, due to the distribution being too flat, not bell-shaped, and the probability plot shows that the data values occur in groups, raising concerns about the continuous nature of the measurements. This could potentially be analyzed as is, since the W is close to 0.90, but the data should be examined carefully for errors and scientific meaning. If the data are categorical, then alternative statistical methods should be considered.

Peaked Stem Leaf

 next >> ( Step D. Check for model validity )



  H I N T S :
  Notice the asterisks in the second boxplot. That usually indicates outliers, but in this case, it is due to the non-normality. Thus, you might want to check normality before discarding outliers.
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