Cutting my wisdom teeth

“A man begins cutting his wisdom teeth the first time he bites off more than he can chew” Herb Caen

This was very much my experience when teaching about ’Study Designs’ on day one of the Practice of Evidence Based Health Care. Active, enthusiastic and engaged audiences, in my opinion, always make for a better teaching (and hopefully learning) experience, and so it proved with this years cohort. There were  informative discussions around the best type of study designs, with the group demonstrating their understanding of the fact that in the end it all depends on the question being asked.

I particularly enjoy experiences where I learn something new and an interesting comment from one of the delegates about differences between retrospective (looking back in time) cohort and case control studies provided this. There was an interesting, lively and active debate before we arrived at our answer; it’s all about EXPOSURES and OUTCOMES. In a retrospective cohort study we have a group of people who have already had an exposure but we do not know the outcome(s) of having the exposure. So we look backwards to see what outcome(s) occurred as a result of having the exposure.

An example helped here. Suppose a group of patients have had  an inappropriate surgery (exposure) some time ago. We could look back at the time since their surgery until the present day and note what complications or ill effects (outcomes), if any, occurred. If we want to add a control group, we could take a similarly matched group of individuals who did not receive the inappropriate surgery (i.e. they had the appropriate surgery) and we can compare the complications or ill effects that occurred in this group. We then compare this to our exposed group.

In a case control study the difference is that we have a group of patients who already have the outcome and what is missing is information about the exposure. In our surgery example we would now have a group of patients presenting with a complication (outcome). We would then look back in time to see if they had ever received the inappropriate surgery (exposure). We would need to match this group with a similar group of patients who do not present with complications and see if they had ever received the inappropriate surgery. We then compare the two groups.

Retrospective cohort studies might seem rare but in fact it would appear they have a rich history in medical research. Anyone wishing to know more about this I recommend a very informative paper by Prof. Richard Doll which can be found here:

It was nice to meet this years group and I always enjoy hearing where they come from, their different backgrounds and their differing reasons and aspirations for coming to Oxford. It’s also nice to be part of a team that make it our aim to try and meet these aspirations. I wish this years cohort a pleasant stay in Oxford but more so I hope they return enthused and eager to put evidence into practice.

About David Nunan

Postdoctoral research fellow, University of Oxford and BASES accredited Exercise Physiologist.

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