A group of investigators reported on the survival of women treated for breast cancer at their institution from 1935 to 1982: Over the period of the study, no significant change was noted in overall survival. However, survival among patients with metastatic disease at the time of diagnosis of breast cancer improved substantially.

The median age-adjusted survival of such patients in the period 1934–1954 was six months; in the period 1955–1974 it increased to 21 months; and in the period 1975–1982 it increased further to 28 months.

This increased survival is probably related to advances in therapy, such as combination chemotherapy. Apart from chance, what is yet another explanation for the findings regarding the change over time in survival among women with metastatic disease?