Harvard researchers report that avoiding weight gain may be an important factor in reducing breast cancer in post-menopausal women. This study certainly can't be considered reliable evidence of that. In yet another epidemiologic study based on the data tragedy known as the Nurses Health Study, the researchers reported that women who gained more than about 40 pounds since age 18, had a 40 percent increase in breast cancer incidence. The reason for this is simple.
Epidemiologic data are likely to be imprecise.
The Nurses Health Study is a great example of this. The Nurses Health Study is a cohort of more than 95,000 nurses followed since 1976. The nurses filled out an initial health questionnaire in 1976, and follow-up questionnaires have been mailed every two years since. But the questionnaire responses have not been validated or verified. A sufficient amount of bad data may give rise to spurious results.