Changes in lifestyle plays an important role in recovery from
heart disease . A doctor wants to encourage the patients to
exercise more by organizing a free weekly exercise class and
sending invitation cards to patients so that they would enroll if
they like. After five years, the results show that the
patients who came to the class had significantly better
cardiovascular health than those who didn't come to the class.
Mark all that is true ( can be more than one answer)
This is an observational study.
We can't infer class participation caused health improvement,
because of s election on the dependent variable.
If invitation cards are sent to a randomly-selected fraction of
patients, then we can conclude exercise class caused health
improvement.