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 The POWER Procedure
 Error and Information Output

The Error column in the main output table provides reasons for missing results and flags numerical results that are bounds rather than exact answers. For example, consider the sample size analysis implemented by the following statements:

```proc power;
twosamplefreq test=pchi
method=normal
oddsratio= 1.0001
refproportion=.4
nulloddsratio=1
power=.9
ntotal=.;
run;
```

Figure 68.6 Error Column
The POWER Procedure
Pearson Chi-square Test for Two Proportions

Fixed Scenario Elements
Distribution Asymptotic normal
Method Normal approximation
Null Odds Ratio 1
Reference (Group 1) Proportion 0.4
Odds Ratio 1.0001
Nominal Power 0.9
Number of Sides 2
Alpha 0.05
Group 1 Weight 1
Group 2 Weight 1

Computed N Total
Actual Power N Total Error
0.206 2.15E+09 Solution is a lower bound

The output in Figure 68.6 reveals that the sample size to achieve a power of 0.9 could not be computed, but that the sample size 2.15E+09 achieves a power of 0.206.

The Info column provides further details about Error column entries, warnings about any boundary conditions detected, and notes about any adjustments to input. Note that the Info column is hidden by default in the main output. You can view it by using the ODS OUTPUT statement to save the output as a data set and the PRINT procedure. For example, the following SAS statements print both the Error and Info columns for a power computation in a two-sample test:

```proc power;
twosamplemeans
meandiff= 0 7
stddev=2
ntotal=2 5
power=.;
ods output output=Power;
run;
```
```proc print noobs data=Power;
var MeanDiff NominalNTotal NTotal Power Error Info;
run;
```

The output is shown in Figure 68.7.

Figure 68.7 Error and Info Columns
MeanDiff NominalNTotal NTotal Power Error Info
0 2 2 . Invalid input N too small / No effect
0 5 4 0.050   Input N adjusted / No effect
7 2 2 . Invalid input N too small
7 5 4 0.477   Input N adjusted

The mean difference of 0 specified with the MEANDIFF= option leads to a "No effect" message to appear in the Info column. The sample size of 2 specified with the NTOTAL= option leads to an "Invalid input" message in the Error column and an "NTotal too small" message in the Info column. The sample size of 5 leads to an "Input N adjusted" message in the Info column because it is rounded down to 4 to produce integer group sizes of 2 per group. Previous Page | Next Page | Top of Page