Thanks for your help with my questions about my case-control study. Because of some workload challenges within our department, my supervisor needs me to work with another team that is currently involved in a retrospective cohort study. My job on the team is to look at some of the results of the study and make any recommendations about whether or not we should continue to research the issue or if the results are sufficient as is and we should conclude this section of the study.

Here’s what I know so far. My team has been looking at the town of Uintah, which borders Mt. Carmel, a volcano that erupted 15 years ago. Because of the hazardous gases and ash emitted during an eruption, specifically the gases hydrogen sulfide and sulfur dioxide, we are looking to find out if living in this town at the time of eruption is associated with higher rates of lung cancer, as well as other outcomes. My team has been comparing two groups:

• Group A (exposed) - those who lived in the town of Uintah when Mt. Carmel erupted 15 years ago.
  
• Group B (unexposed) – those who lived in the town of Delta when Mt. Carmel erupted 15 years ago.

We looked at historical records to identify everyone who had lived in the two towns at the time of the eruption and followed up with them to see if they had been diagnosed with lung cancer. In some instances they were already deceased, so we contacted family members and looked at death certificates to get the information. The people who were still living were contacted directly. We did the same thing in Delta, a town of similar size to Uintah but that is not close to a volcano. Because age is a confounding factor with cancer, it was important that we compared people in the same age cohorts. The team used questionnaires for the people who were still alive and the families of the deceased asking, among other things, whether or not they had been diagnosed with lung cancer. Overall we had a very good response rate. The team also wanted to look at gender to see if there was a difference between the two, so they looked at males and females separately.

I’ve been asked to summarize the findings we have so far for my supervisor and to make a recommendation about whether or not we should fund follow-up studies. But in going through the research notes about this study, I’m now confused by some of the information I’ve found. Can you help me sort this out? Here are the questions I have for you:

1. Why did they do a cohort study here instead of a case control study?
   
2. What is the cohort in this case?
   
3. What do these results tell me about the people who live in Uintah and their risk for lung cancer, if there is a risk? (The RR=3.9 for women and RR=9.1 for men, but I’m not sure what this tells me.)
   • Should I report the RR for each gender separately, or calculate the total?

Thanks,
Andy