Friday, 5 September 2014

Different Study Types

A couple of people commented on my last post that they sometimes find it difficult to critically analyse a primary research paper or necessarily trust it. Here is a short list of the different study types and their various advantages and disadvantages.

Studies are normally divided into what we term descriptive studies or analytic studies. So there isn’t an information overload, I will just discuss descriptive studies on this post and will discuss analytic studies on my next post.

Descriptive studies on a population level are termed ‘ecological studies’ and on an individual level are termed ‘case reports’, ‘case series’ or’ cross sectional studies’.

A good example of an ‘ecological study’ was that conducted by Friedman and colleagues in 1967 which found that deaths from coronary heart disease in 44 US states correlated with the number of cigarettes sold in those states; i.e. the states selling the most cigarettes had the highest levels of coronary heart disease.  The advantages of such a study are that they are helpful in finding correlations between risk factors and disease outcomes. The disadvantages are that we do not get information about individuals so it is impossible to tell if the people with the risk factor (i.e. those buying the cigarettes)  are the ones getting the disease (coronary heart disease). This is termed the ‘ecological fallacy’. The other problem is ‘confounding’ – i.e. there may be other risk factors at play which aren’t being considered.

Case reports and Case series are examples of ‘ecological studies’ on an individual level. A good example of this is the case series which emerged from the Centre for Disease Control and Prevention in 1981 reporting the occurrence of the rare lung infection PCP (Pneumocystic carinii pneumonia) in a group of 5 otherwise healthy men in LA. This heralded the beginning of awareness of AIDS in the USA. Relevant to MS are the case reports that came out around 2005 showing that natalizumab (Tysabri) can result in JC virus induced PML .

The important thing to take into account when looking at a case series is to ensure that the patient’s being reported are representative of all the patients with the condition so that conclusions can be generalised. The advantages of such studies are that they are good for forming hypotheses and describing clinical experience. It is also a good way to inform the medical community of the first cases of what could be an emerging condition. The disadvantages are that the finding may be pure chance or the patient selection may be biased making generalisations difficult.

The final example of an ‘ecological study’ is the ‘cross sectional study’ also termed a ‘prevalence study’. The object of this design is to estimate how common a disease is within a population. In reality, one can’t test the whole population so a sample of the population is taken. The advantages of such studies are that they are good for common diseases (like high blood pressure/high cholesterol) as you only need a small sample and the study is normally of short duration. It’s not useful for rare diseases as they are less likely to be accurately represented in a sample of the population. The other point to take into account is that those who normally volunteer for such studies tend to be well educated and health conscious so they may not be representative of the population – we term this ‘confounding’.

I’ll post about analytical studies in the next couple of days but you can see the links to the studies I mentioned below. 

Cigarette smoking and geographic variation in coronary heart disease mortality in the United States
http://www.sciencedirect.com/science/article/pii/0021968167900896

PCP Case Series
http://www.cdc.gov/mmwr/preview/mmwrhtml/june_5.htm

Progressive Multifocal Leukoencephalopathy after Natalizumab Therapy for Crohn's Disease


1 comment:

  1. Thanks, Arie. This is a great summary of the study types.

    A major combination ecological study and cross sectional study are the apparent correlations between Vitamin D and MS occurrence. This is a perfect example of something where - to my knowledge - the causal link is unclear, but many neurologists advise their patients to take Vitamin D supplements since the downside is so low.

    One other relevant ecological case series is the reports on PML occurring in patients taking dimethyl fumarate. Professor G did some great posts that highlighted why these particular cases were not representative of the entire population. As a recent Tecfidera convert - let's hope this is the case!

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