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Interim review of the transnational case‐control study of oral contraceptives and health: Approved protocol revisions through September 1995

The protocol of the Transnational Study on Oral Contraceptives has been published at the outset of the study in order to ensure the correct performance of the study and to ensure the study's scientific integrity. Potential revisions and additions to this protocol had been announced in the prior publication. This article focuses on those points of protocol which have been resolved as practical experience was gathered as well as additional modifications required by practical and scientific considerations. The major alterations are based on exposure prevalences in controls and on power calculations. The low (1.8%) exposure to gestodene among German controls led to the discontinuation of the German component and to the accelerated accession in additional countries. The case:control ratio was altered to 3:1 for purposes of economy based on power calculations. Numerous minor points, most addressed in the prior protocol, have been resolved. It is found that these modifications of the study protocol do not impair the study's conduct, its scientific integrity, nor its capability of providing answers to the study question.