There have been people enough to sell their own children for the right price.

The answer to this question is Mercenary
Mercenary refers to an establishment that agree to do a certain task as long as the requester provide them with appropriate payment.
Mercenary often do not run on principle and they believe that loyalty only became a hindrance for their operation.

Cohort

Explanation:

Cohort studies refer to a form of medical study which is used to examine the reasons for the disease and to find its relation to the risk factors to medical outcomes. Usually, cohort studies track large groups of people, tracking their susceptibility to such risk variables in order to obtain answers to the potential causes of disease.  

As per the question, researchers observe a group of people based on their smoking habits and tracked them for twenty years to observe its relation to lung cancer reflects the procedure of cohort studies.