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

BACKGROUND

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 has been promoted to correct for this bias, 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.

OBJECTIVE

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

METHODS

A first step quantified self-selection and 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 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.

RESULTS

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 quantity required to correct results of observational studies was 1.53. In 19 case-control and cohort studies on screening mammography effectiveness, the median quantity used for correction purposes was 1.16 (IQR: 1.11-1.28).

CONCLUSION

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 quantities that were too small to correct for self-selection.