English: An example of a group sequential design, shown in terms of the test statistic, a standardised measure of the treatment effect. The trial design has three analyses, shown on the x axis. The upper, green line is the boundary for stopping for success; if, at an analysis, the test statistic is greater than this boundary, the trial will stop for success. On the other hand, if, at an analysis, the test statistic is less than the lower, red boundary, the trial will stop for futility. If the test statistic is between the two boundaries, more participants are recruited and the trial continues until at least the next analysis. For the final stage, the upper and lower boundaries meet, so that the trial will definitely end. In this example, the test statistic for the first analysis is between the boundaries, so the trial continues. At the second analysis, the test statistic is greater than the upper boundary, so the trial stops for success.