Methods
During the INSECT trial, 625 patients were randomized at 25 surgical sites between July 11, 2004, and September 26, 2006. The detailed study protocol of the INSECT trial describes several strategies to ensure high internal validity including primary hypothesis, randomization, sample size calculation, harmonization and standardization of treatment, assessment and bias. In addition, issues that potentially affect external validity were recorded, namely detailed information describing the process from screening to randomization in two participating centers.
Setting of the Trial
The types of all participating surgical departments were categorized into primary, secondary or tertiary care centers. Primary care centers (PCC) were defined as hospitals with basic diagnostic and therapeutic options offering general surgical interventions. Secondary care centers (SCC) were hospitals with a broader surgical spectrum (i.e. more specialized departments) and advanced diagnostic and therapeutic tools to which a patient had been referred by primary care providers. Tertiary care centers (TCC) act as referral centers and are often academic hospitals with the highest standards of care, including access to most specialists and the necessary equipment that may be lacking in primary and secondary care centers. The selection of participating centers and surgeons was purely based on the self-motivation of the according centers. Once they agreed to adhere to the protocol, signed a contract, received ethical approval from their local ethics committee and finalized training, they were able to start patient recruitment. In order to assess whether this process caused imbalances between the patient populations at the different hospital levels, analyses concerning the recruitment of patients and demographics (i.e. age and gender) as well as clinical parameters (body mass index) and surgery-related parameters (type of procedures) were performed. In addition, the level of expertise of surgeons and the adherence to the protocol were assessed.
Selection of Patients
Inclusion and exclusion criteria of the trial were shown previously. Due to the incomplete recording of screening lists, the according data was not available from other participating centers. Patients at the Department of Heidelberg were analyzed for demographics (age and gender), clinical parameters (body mass index, ASA score, duration of hospitalization, presence of malignant diseases), and surgery-related parameters (incision technique, closure technique) according to three specified subgroups: included patients, eligible patients who declined participation and eligible patients randomized to one other RCT.
The random ratio was calculated by dividing the number of randomized patients/participants by the number of primarily eligible patients/eligible for participation for the Department of Surgery of the University of Heidelberg and Erlangen. The random ratio thus equals the enrollment fraction as described by Gross et al.. According to Gross et al. and Jones et al., the enrollment fraction equals the proportion of people who are eligible for participation and who actually enroll in the RCT.
Data analysis was done using SAS™ 9.1 Win (Release 9.1, SAS Institute, Inc., Cary, NC). A description of the data included absolute and relative frequencies for categorical data and mean and standard deviations for continuous data. Possible differences between groups were calculated with Fisher's exact test of categorical parameters, Kruskal-Wallis test for continuous variables, and binomial test for proportion of randomized patients. The explorative statistical significance level was set at 5% (0.05); no adjustment for multiple comparisons was done due to the descriptive nature of the study.
This article was structured based on the recommendations according to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) in order to include an accurate and complete report of an observational study. The original INSECT trial was published in accordance with the CONSORT criteria.