sample size











Role of Sample Size - Precision

Power as a Function of Effect Size and N - Two sample proportionsThe confidence interval represents the precision with which we are able to report the effect size, and the larger the sample, the more precise the estimate. As a practical matter, sample size is often the dominant factor in determining the precision.

Figure 3 shows precision for a rate difference as a function of sample size. This figure is based on the same rates used in the Power analysis (30% vs. 50%). With N=50 per group the effect would be reported as 20 points with 95% confidence interval of plus/minus some 19 points (01 to 39 points). With N=100 per group the effect would be reported as 20 points with 95% confidence interval of plus/minus some 13 points (7 to 33). With N=200 per group the effect would be reported as 20 points with 95% confidence interval of plus/minus some 9 points (11 to 29).

Note: For studies that involve two groups precision is maximized when the subjects are divided evenly between the two groups (this statement applies to the procedures included in this program). When the number of cases in the two groups is uneven the "effective N" for computing precision falls much closer to the smaller sample size than the larger one.

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