Applicable One-Sample Tests and Sample Size Computation |
The SEQDESIGN procedure provides sample size computation for two one-sample tests: normal mean and binomial proportion. The required sample size depends on the variance of the response variable—that is, the sample proportion for a binomial proportion test.
In a typical clinical trial, a hypothesis is designed to reject, not accept, the null hypothesis to show the evidence for the alternative hypothesis. Thus, in most cases, the proportion under the alternative hypothesis is used to derive the required sample size. For a test of the binomial proportion, the REF=NULLPROP and REF=PROP options use proportions under the null and alternative hypotheses, respectively.
The MODEL=ONESAMPLEMEAN option in the SAMPLESIZE statement derives the sample size required to test a normal mean by using the sample mean statistic for the null hypothesis . At stage , the sample mean is computed as
where is the value of the th observation available in the th stage and is the cumulative sample size at stage .
An equivalent hypothesis is , where .
The MLE statistic for ,
where the information
is the inverse of the variance.
That is, the standardized statistic
Thus, to test the hypothesis against a two-sided alternative , is rejected at stage if the statistic is less than or equal to the lower boundary value or if is greater than or equal to the upper boundary value at stage .
If the variance is unknown, the sample variance can be used if it is assumed that the sample variance is computed from a large sample such that the test statistic has an approximately normal distribution.
The maximum information is needed to derive the required sample size. If the maximum information is not specified or derived with the ALTREF= option in the procedure, the MEAN= option in the SAMPLESIZE statement is used to specify the alternative reference and thus to derive the maximum information.
In the SEQDESIGN procedure, the computed total sample size
where is the maximum information and is the specified standard deviation. With an available maximum information, you can specify the MODEL=ONESAMPLEMEAN( STDDEV= ) option in the SAMPLESIZE statement to compute the required total sample size and individual sample size at each stage. A procedure such as PROC MEANS can be used to derive a one-sample test for a normal mean.
The MODEL=ONESAMPLEFREQ option in the SAMPLESIZE statement derives the sample size required to test a binomial proportion by using the null hypothesis , where is the proportion of a binomial population. At stage , the MLE for is computed as
where is the value of the th observation available in the th stage and is the cumulative sample size at stage .
An equivalent hypothesis is , where . If is not close to or , then for a large sample, has an approximately normal distribution
where the information is the inverse of the variance .
Then the standardized statistic
In practice, the estimated sample proportion at stage can be used to derive the information and test statistic . Thus, to test the hypothesis against an upper alternative , is rejected at stage if the statistic is greater than or equal to the upper boundary at stage .
The maximum information is needed to derive the required sample size. If the maximum information is not specified or derived with the ALTREF= option in the procedure, the PROP= option in the SAMPLESIZE statement is used to specify the alternative reference and to derive the maximum information for the sample size calculation.
It is assumed that the sample size is sufficiently large such that the test statistic has an approximately normal distribution. With the hypotheses and , the SEQDESIGN procedure derives the total sample size
where if REF=NULLPROP is specified. Otherwise, .
If the PROP= option in the SAMPLESIZE statement is not specified, then the alternative reference derived in the SEQDESIGN procedure is used to compute .
The ALTREF= option in the PROC statement can be used to specify . Otherwise, the PROP= option in the SAMPLESIZE statement must be specified.
For example, with , , and REF=PROP (which is the default),
You can specify the MODEL=ONESAMPLEFREQ option in the SAMPLESIZE statement to compute the required total sample size and individual sample size at each stage. A procedure such as PROC GENMOD with the default DIST=NORMAL option in the MODEL statement can be used to derive the test for a binomial proportion.