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Basic Analysis and Graphing - SAS

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Chapter 5 Performing Oneway <strong>Analysis</strong> 143<br />

Means/Anova <strong>and</strong> Means/Anova/Pooled t<br />

The Block Means Report<br />

If you have specified a Block variable on the launch window, the Means/Anova <strong>and</strong> Means/Anova/Pooled<br />

t comm<strong>and</strong>s produce a Block Means report. This report shows the means for each block <strong>and</strong> the number of<br />

observations in each block.<br />

Mean Diamonds <strong>and</strong> X-Axis Proportional<br />

A mean diamond illustrates a sample mean <strong>and</strong> confidence interval.<br />

Figure 5.9 Examples of Mean Diamonds <strong>and</strong> X-Axis Proportional Options<br />

overlap marks<br />

95%<br />

confidence<br />

interval<br />

group mean<br />

x-axis proportional<br />

x-axis not proportional<br />

Note the following observations:<br />

• The top <strong>and</strong> bottom of each diamond represent the (1-alpha)x100 confidence interval for each group.<br />

The confidence interval computation assumes that the variances are equal across observations.<br />

Therefore, the height of the diamond is proportional to the reciprocal of the square root of the number<br />

of observations in the group.<br />

• If the X-Axis proportional option is selected, the horizontal extent of each group along the x-axis (the<br />

horizontal size of the diamond) is proportional to the sample size for each level of the X variable.<br />

Therefore, the narrower diamonds are usually taller, because fewer data points results in a wider<br />

confidence interval.<br />

• The mean line across the middle of each diamond represents the group mean.<br />

• Overlap marks appear as lines above <strong>and</strong> below the group mean. For groups with equal sample sizes,<br />

overlapping marks indicate that the two group means are not significantly different at the given<br />

confidence level. Overlap marks are computed as group mean ± ( 2) ⁄ 2 × CI ⁄ 2 . Overlap marks in one<br />

diamond that are closer to the mean of another diamond than that diamond’s overlap marks indicate<br />

that those two groups are not different at the given confidence level.<br />

• The mean diamonds automatically appear when you select the Means/Anova/Pooled t or Means/<br />

Anova option from the platform menu. However, you can show or hide them at any time by selecting<br />

Display Options > Mean Diamonds from the red triangle menu.

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