It has been suggested that occupational exposure to benzene in the petroleum industry increases the risk of developing leukemia. the levels of benzene to which workers in this industry have been exposed were high from 1940 to 1970, but since 1970 have been significantly reduced. what kind of study design, using petroleum workers, would provide the most useful information on whether benzene affects incidence rates of leukemia in this industry? you may assume that records of individual worker assignments to jobs involving benzene exposure have been maintained by the industry.

(a) experimental
(b) retrospective cohort
(c) prospective cohort
(d) case-control
(e) cross-sectional

(B) Retrospective cohort                  

Explanation:

Retrospective cohort studies: The retrospective cohort studies are one of the types of observational research studies including the researchers to look back in time or past at self-report or archived data to investigate the difference in the risk of disease between exposed and non-exposed patients.

Cohort studies are better than case-control studies because they are generally prospective. The cohort studies are inexpensive to carry-out.

In the question above, the statement signifies the retrospective cohort study.

1. state-issued photo ids. provide them for free for those who can't afford them.

2. the system we have seems to be working nearly perfectly. seems like about 31 cases per billion votes cast

3. the national policy for hiring new employees which requires proof of eligibility to work. there are something like 20 types of identification which can be used for this.