Re: Indoor tanning and non-melanoma skin cancer: systematic review and meta-analysis
We thank Dr. Tunnicliff for her thoughtful comments. We agree that it was important to use a very conservative method to synthesize these observational studies that were predominantly small, retrospective, and case-control in design. We used a random-effects model to accomplish this goal; the χ2 p-values and I2 values were included for completeness but did not affect our choice of model.
Dr. Tunnicliff mentioned that some original studies were "less than optimum" and certainly conclusions of meta-analyses are always limited by the quality of included studies. We focused on what we judged to be the most significant sources of bias when combining effect estimates: lack of consistency in the exposure measurement and differences in adjustment for confounders. We combined studies based on a consistent exposure measure: 'ever exposure to indoor tanning.' As we noted, the primary analysis excluded the 2 of 12 studies that did not report an ever-exposure estimate. Also, the conclusions did not change in sensitivity analyses that excluded studies that did not fully adjust for confounders.
As Dr. Tunnicuff mentioned, observational data from different studies should be combined with caution, and we believe our conservative approach represents the most accurate synthesis of results currently available.