Statistical issues related to early closure of STOP-ROP, a group-sequential trial

The Supplemental Therapeutic Oxygen for Prethreshold Retinopathy of Prematurity trial used the group-sequential alpha-spending approach with asymmetrical tails to investigate whether supplemental oxygen therapy would reduce the proportion of infants progressing from prethreshold retinopathy of prematurity (ROP) to threshold ROP from 30% to 20%. Three years of enrollment were predicted. After 4 years, the data and safety monitoring committee (DSMC), faced with a projected delay of 2-3 more years, opted to terminate the trial after 1 further year of enrollment despite a continually borderline test statistic. We discuss factors relating to the DSMC decision and its effect upon the conclusions that may be drawn from the trial.