Around 2002, after several decades of steady increase, the incidence of breast cancer in many parts of the developed world began to decrease.1 2 Several explanations for this decline have been proposed, but a favoured theory has been a reduction in the use of hormone replacement therapy (HRT) after release of the results of the Women’s Health Initiative randomised trial.1 2 3 4 In the linked study (doi:10.1136/bmj.e299), Weedon-Fekjær and colleagues present ecological data supporting a role for mammography screening and HRT use in recent trends in the incidence of breast cancer.5

Ecological studies use aggregate data to explore correlations and time trends. No information is available on individuals, so no inferences can be drawn about cause and effect. Thus, Weedon-Fekjær and colleagues’ study provides no data on whether the women who participated in the screening programme were the same women identified with breast cancer, or whether women who used HRT developed breast cancer more often than those who did not. Instead, the correlations reported are …