Self-Selection Bias in Randomized and Observational Studies on Screening Mammography: A Quantitative Assessment

Observational studies aimed at evaluating the effectiveness of screening mammography are prone to self-selection due to differences in personal characteristics between women attending and those not attending screening. A method based on a quantity Dr has been promoted to correct for this bias, Dr being the risk of breast cancer death in a group of women not attending screening compared to the risk of breast cancer death in a population without screening.

To estimate the amount of self-selection in observational studies aimed at evaluating screening mammography effectiveness and to estimate Dr quantities needed to correct for this bias.

A first step quantified self-selection and Dr quantities specific to Swedish randomized trials using the most recent publications. A second step estimated self-selection specific to cohort studies on screening mammography effectiveness using the relative risk of 0.54 for all-cause death from these studies and the relative risk of all-cause death of 0.98 reported in Swedish trials. Using self-selection estimated from cohort studies, the Dr quantity needed to correct observational studies on screening mammography effectiveness was estimated. In a last step, corrections for self-selection in observational studies on screening mammography were retrieved.

The self-selection bias was 2.10 in Swedish trials. Self-selection in cohort studies was computed as (0.98/0.54) = 1.78. The Dr quantity required to correct results of observational studies was 1.53. In 19 case-control and cohort studies on screening mammography effectiveness, the median Dr quantity used for correction purposes was 1.16 (IQR: 1.11-1.28).

Compared to women attending screening, the risk of breast cancer death was approximately two times greater in women not attending screening. This increased risk was independent of screening effects. Most observational studies have overestimated the effectiveness of screening mammography because they used Dr quantities that were too small to correct for self-selection.