Comparability Criteria Selection
When insufficient data exists to power a statistical equivalency test such as TOST, the TI method may be considered an alternative. In general, a useful technique to consider the adequacy of each statistical approach is to perform a statistical performance assessment (SPA). An SPA permits an early assessment of each technique in order to correctly show comparability when there is no difference across means and correctly show noncomparability when there is a difference across means.
As an illustration of the TOST and TI approaches, consider the following scenario: 12 values are sampled from the pre-change process, and four values are sampled from the post-change process.
This data will be used to establish comparability in means. To calculate the SPA, we shall further assume the following:
1. Pre- and post-change processes are normally distributed,
2. Pre- and post-change processes have equal variances,
3. The TOST goalpost is two times the standard deviation from the pre-change process, and
4. A 90/99% tolerance interval will be calculated using pre-change process data.
It is possible to calculate the probability of meeting the comparability criteria by calculating the statistical power for a TOST approach, and the probability of all four post-change values falling inside the TI. The results are shown in Table 2 and the Figure.