Types of descriptive and analytical studies

In my last post, I talked about what descriptive and analytical studies are. I also mentioned what types of studies fall under these two categories.

So quick recap

1. Observational studies consists of descriptive and analytical studies.
2. This can divided further into 6 different types of study designs.

I think the easiest way to approach this is to explain each type ending with the two that overlap – that is Ecological and Cross-sectional study. Hopefully, you will understand why by the end of the of this post.

Descriptive studies

1. Case Report
   These are usually common in medicine. It basically is an interesting or rare case that is written up for the purpose of letting other people learn more about it. It details the symptoms, signs, diagnosis, how we got to the diagnosis, management and follow-up. It is based on an individual scenario.
   
2. Case Series
   Can be thought of as multiple case reports , a collection of information or a group of people all of which are unified by a single exposure. For example, you have a number of patients given a particular treatment. You then review their records to establish the exposure and the outcome.

NOTE – Both case series and case reports are observational. What we see is literally what we write up and document. The exposures and outcomes are not controlled or augmented. Think of it as a journalist reporting on an event.

Analytical Studies

3. Case-control studies – remember here we have a hypotheses and we are testing it!

First of all we need to establish that in case-control studies we are looking back in time/retrospectively. We have our data and we establish two different groups. Our Case group consists of individuals with a particular outcome. Our Control group consists of individuals without the outcome. After establishing our groups, we look back on both and see how they differ in being exposed to a certain risk factor that we think contributes to the outcome. A classic example is the influenza virus and vaccinations. What are we trying to assess – whether vaccines reduce the chance of a person getting influenza.

Case – people with influenza
Control – people without influenza
What do we think is the reason they did not get ill in this instance? Vaccines!

Also, because it is retrospective it is relatively cheap and easy to carry out without losing patients to follow-up. However it tells us nothing about the incidence of the disease in the population and there might be some recall bias – as we look back in time.

4. Cohort studies

Cohort studies are carried out in the present. Once again you have two groups (let’s use the influenza example) one group who has been vaccinated while another has not. We then follow-up this group in real time to see who gets the influenza virus. What we are assessing is an outcome based on whether they were exposed to a risk factor. Another way of looking at this is the risk factor being the human papilloma virus (HPV) and the outcome being cervical cancer. So we essentially follow up these two groups to see how many from each group get cervical cancer and if there is a correlation to HPV infection.

Now, because this is done in real time it is not as easy or cheap to carry out. There is also a high chance of losing people to follow-up. However, it does tell you the incidence of a certain outcome in each group and there is less chance for recall bias.

Descriptive & Analytical

Ecological & Cross-sectional study

Now, the first thing you need to know, is that ecological and cross-sectional studies are very similar. They both basically give us a snapshot of a population at a point in time or over a certain period (ie a year). So what they tell us is the prevalence (how much of a disease there already is) at any given time/period of time. The defining difference is that ecological studies focuses on an entire population, while cross-sectional can be at a population level or a subset of it.

So when going over data, you will never see individuals in an ecological study but groups or aggregates of individuals based on things such as incomes, countries, age groups etc. While cross-sectional studies focus on the individuals.

These studies are both done in real time, and can be used to both test or generate a hypothesis therefore being both descriptive and analytical.

How?

An ecological study can be looking at the different income groups and their health at a given time and a cross-sectional study can just look at the people who have a particular disease at a given time. We just get the facts and learn about the prevalence of a certain disease. We then can generate a hypothesis and carry out an analytical study to test the association by doing either

If we were doing a descriptive study about chronic lung disease in Malaysia, we would look at all the states in Malaysia and gather info on which has the highest amount of chronic lung disease. But then we think, I wonder if the haze in South East Asia has an association with chronic lung disease? We then do an analytical ecological study, looking at the different states with chronic lung disease and their air pollution indexes. However, because it is an ecological study it really does not tell us anything about the individual, for all we know there could be other risk factors such as them smoking, having previous nasty chest infections etc.

For cross-sectional studies, a descriptive study would be a survey telling us the prevalence of hypertension in a population. All we have here is the how many people have the disease. Let’s say we think aging affects your chances of getting hypertension. We then look at people with hypertension and without hypertension and see if old age has a correlation. Here we have individual data instead of groups.

What is really important about both studies is that it only tells us about what cases are already there (prevalence) but nothing about new cases (incidence). It is because of this it doesn’t really help with studying cause and effect.

I know this is a ridiculously long post, but I hope it has been beneficial and I have managed to explain this as easily as possible. It did take me a couple of tries to wrap my head around it. Do leave me any feedback or thoughts, would love to hear from you!