6afb5300_usation___HealthNewsReview_org__Study_design
Long-term daily use of adult-strength aspirin may be associated with modestly reduced overall cancer incidence Higher aspirin dose seems to stave off some cancers The strongest effect was for colon cancer. [Problem] Stave off is causal and active; effect is causal. Seems to, used as a caveat, does not undo the implication of causality. [Suggested language] Because the study was based on observation rather than a true experiment, we still dont know whether aspirin truly had a protective effect against cancer. A randomized trial would be needed to prove that causal link. [Study design] 
Prospective cohort study of aspirin use and cancer incidence among U.S. men and women (observational)
Observational Studies - Does The Language Fit The Evidence? - Association Versus Causation - HealthNewsReview.org
Study design