What are strategies in the study design and data collection

Solution

The definition of risk and outcome should be clearly defined prior to study implementation. Subjective measures, such as the Baker grade of capsular contracture, can have high inter-rater variability and the arbitrary cutoffs may make distinguishing between groups difficult . This can inflate the observed variance seen with statistical analysis, making a statistically significant result less likely. Objective, validated risk stratification models such as those published by Caprini and Davison for venous thromboembolism, or standardized outcomes measures such as the Breast-Q should have lower inter-rater variability and are more appropriate for use. When risk or exposure is retrospectively identified via medical chart review, it is prudent to crossreference data sources for confirmation. For example, a chart reviewer should confirm a patient-reported history of sacral pressure ulcer closure with physical exam findings and by review of an operative report; this will decrease discrepancies when compared to using a single data source.

Data collection methods may include questionnaires, structured interviews, physical exam, laboratory or imaging data, or medical chart review. Standardized protocols for data collection, including training of study personnel, can minimize inter-observer variability when multiple individuals are gathering and entering data. Blinding of study personnel to the patient\'s exposure and outcome status, or if not possible, having different examiners measure the outcome than those who evaluated the exposure, can also decrease bias. Due to the presence of scars, patients and those directly examining them cannot be blinded to whether or not an operation was received. For comparisons of functional or aesthetic outcomes in surgical procedures, an independent examiner can be blinded to the type of surgery performed. For example, a hand surgery study comparing lag screw versus plate and screw fixation of metacarpal fractures could standardize the surgical approach (and thus the surgical scar) and have functional outcomes assessed by a blinded examiner who had not viewed the operative notes or x-rays. Blinded examiners can also review imaging and confirm diagnoses without examining patients.

Selection bias

Selection bias may occur during identification of the study population. The ideal study population is clearly defined, accessible, reliable, and at increased risk to develop the outcome of interest. When a study population is identified, selection bias occurs when the criteria used to recruit and enroll patients into separate study cohorts are inherently different. This can be a particular problem with case-control and retrospective cohort studies where exposure and outcome have already occurred at the time individuals are selected for study inclusion . Prospective studies (particularly randomized, controlled trials) where the outcome is unknown at time of enrollment are less prone to selection bias.

Channeling bias

Channeling bias occurs when patient prognostic factors or degree of illness dictates the study cohort into which patients are placed. This bias is more likely in non-randomized trials when patient assignment to groups is performed by medical personnel. Channeling bias is commonly seen in pharmaceutical trials comparing old and new drugs to one another . In surgical studies, channeling bias can occur if one intervention carries a greater inherent risk . For example, hand surgeons managing fractures may be more aggressive with operative intervention in young, healthy individuals with low perioperative risk. Similarly, surgeons might tolerate imperfect reduction in the elderly, a group at higher risk for perioperative complications and with decreased need for perfect hand function. Thus, a selection bias exists for operative intervention in young patients. Now imagine a retrospective study of operative versus non-operative management of hand fractures. In this study, young patients would be channeled into the operative study cohort and the elderly would be channeled into the nonoperative study cohort.