6afb5300_usation___HealthNewsReview_org__Study_design
Activity energy expenditure was strongly associated with lower risk of mortality in healthy older adults. For every 287 kcal/day in free-living activity energy expenditure, there is approximately a 30% lower risk of mortality. The authors calculated that participants who did 75 minutes a day of activities lowered their risk of dying by 30% [Problem] Lowered their risk is causal; strongly associated with lower risk is not. [Suggested language] The researchers observed that people who used more energy in daily living had a lower risk of dying (within a certain time period). However, an observational study like this one can’t prove that using more energy in daily activity actually caused the lower risk of dying, because other factors may have played a role. [Study design] 
Prospective cohort study of the relationship between free-living activity energy expenditure and mortality in older adults (observational)
Observational Studies - Does The Language Fit The Evidence? - Association Versus Causation - HealthNewsReview.org
Study design