Page 195 - Prevention and Treatment of Incisional Hernia- New Techniques and Materials

in each group needed to evaluate the primary endpoint. All studies have “dropouts”, because the patients are lost to follow-up, die, or are not willing to continue participation. Therefore, the number of patients to enter in the study should be increased in line with the number of “dropout” patients anticipated, often 10% to 20%.
Interim analysis
Prior to the onset of the study, the protocol of the study should state if an interim analysis will be conducted. An interim analysis is usually done for safety reasons. Therefore, an analysis of the patients “as treated” is the best approach. There are di erent interim analysis procedures and the procedure should be chosen carefully and described in the study protocol.
During an interim analysis the progress of the study inclusions, the occurrence of serious adverse events and the quality of the raw data can also be evaluated. A decision can be made to prolong the inclusion time, to increase the sample size or to stop the trial prematurely. Ideally, an independent data monitoring committee (IDMC) takes such a decision. An example is the study by Itani et al. on ventral hernia repair comparing laparoscopic with conventional surgery(19). The infection rate was so much higher in the conventional group that the data safety monitoring board insisted the trial be stopped.
Description of patient population
The ultimate goal of a study is to generalize the ndings in the study to the larger population of which the study population is a sample. To assess the external validity of a study, the exact method of determining the study sample or study cohort has to be clear.
Mono-centre versus multi-centre studies
There are advantages and disadvantages for both study strategies. Mono- centre interventional studies have a greater chance of having two comparable groups by excluding the variations in the confounding variables that arise from including patients treated in di erent centres. Multi-centre studies have a greater chance of correct inference and generalization of the study results to the larger population in the community. But multi-centre studies are logistically more di cult to perform. Moreover the homogeneity and the quality of the raw data are often inferior in the participating centres compared to the
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Recommendations IH research
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