A network for students interested in evidence-based health care

# An introduction to different types of study design

Posted on 6th April 2021 by

Tutorials and Fundamentals

Study designs are the set of methods and procedures used to collect and analyze data in a study.

Broadly speaking, there are 2 types of study designs: descriptive studies and analytical studies.

## Descriptive studies

• Describes specific characteristics in a population of interest
• The most common forms are case reports and case series
• In a case report, we discuss our experience with the patient’s symptoms, signs, diagnosis, and treatment
• In a case series, several patients with similar experiences are grouped.

## Analytical Studies

Analytical studies are of 2 types: observational and experimental.

Observational studies are studies that we conduct without any intervention or experiment. In those studies, we purely observe the outcomes.  On the other hand, in experimental studies, we conduct experiments and interventions.

### Observational studies

Observational studies include many subtypes. Below, I will discuss the most common designs.

#### Cross-sectional study:

• This design is transverse where we take a specific sample at a specific time without any follow-up
• It allows us to calculate the frequency of disease (prevalence) or the frequency of a risk factor
• This design is easy to conduct
• For example – if we want to know the prevalence of migraine in a population, we can conduct a cross-sectional study whereby we take a sample from the population and calculate the number of patients with migraine headaches.

#### Cohort study:

• We conduct this study by comparing two samples from the population: one sample with a risk factor while the other lacks this risk factor
• It shows us the risk of developing the disease in individuals with the risk factor compared to those without the risk factor (RR = relative risk)
• We may approach this study by 2 longitudinal designs:
• Prospective: we follow the individuals in the future to know who will develop the disease
• Retrospective: we look to the past to know who developed the disease (e.g. using medical records)
• This design is the strongest among the observational studies
• For example – to find out the relative risk of developing chronic obstructive pulmonary disease (COPD) among smokers, we take a sample including smokers and non-smokers. Then, we calculate the number of individuals with COPD among both.

#### Case-Control Study:

• We conduct this study by comparing 2 groups: one group with the disease (cases) and another group without the disease (controls)
• This design is always retrospective
•  We aim to find out the odds of having a risk factor or an exposure if an individual has a specific disease (Odds ratio)
•  Relatively easy to conduct
• For example – we want to study the odds of being a smoker among hypertensive patients compared to normotensive ones. To do so, we choose a group of patients diagnosed with hypertension and another group that serves as the control (normal blood pressure). Then we study their smoking history to find out if there is a correlation.

### Experimental Studies

• Also known as interventional studies
• Can involve animals and humans
• Pre-clinical trials involve animals
• Clinical trials are experimental studies involving humans
• In clinical trials, we study the effect of an intervention compared to another intervention or placebo. As an example, I have listed the four phases of a drug trial:

I:  We aim to assess the safety of the drug (is it safe ?)

II: We aim to assess the efficacy of the drug (does it work ?)

III: We want to know if this drug is better than the old treatment (is it better ?)

IV: We follow-up to detect long-term side effects (can it stay in the market ?)

• In randomized controlled trials, one group of participants receives the control, while the other receives the tested drug/intervention. Those studies are the best way to evaluate the efficacy of a treatment.

## References (pdf)

You may also be interested in the following blogs for further reading:

An introduction to randomized controlled trials

Case-control and cohort studies: a brief overview

Cohort studies: prospective and retrospective designs

Prevalence vs Incidence: what is the difference?

## No Comments on An introduction to different types of study design

• khamisu Habibu

I’m Khamisu Habibu community health officer student from Abubakar Tafawa Balewa university teaching hospital Bauchi, Nigeria, I really appreciate your write up and you have make it clear for the learner. thank you

24th May 2023 at 12:31 pm
• winny

well understood,thank you so much

15th May 2023 at 11:31 pm
• Joyce Mganga

Well understood…thanks

1st February 2023 at 5:21 pm
• Josephine

Simply explained. Thank You.

20th October 2022 at 10:38 am
• Mona Khalil Elias

Thanks a lot for this nice informative article which help me to understand different study designs that I felt difficult before

8th August 2022 at 9:40 pm
• Emma Carter

That’s lovely to hear, Mona, thank you for letting the author know how useful this was. If there are any other particular topics you think would be useful to you, and are not already on the website, please do let us know.

21st September 2022 at 10:53 am
• john

Dear sir

it is very informative and useful.

thank you
statistician

4th August 2022 at 10:33 am
• MIREMBE PATIENCE

Thanks for this information

27th July 2022 at 3:46 pm
• MIREMBE PATIENCE

Thanks so much for this information….I have clearly known the types of study design
Thanks

27th July 2022 at 3:44 pm
• Emma Carter

That’s so good to hear, Mirembe, thank you for letting the author know.

21st September 2022 at 10:54 am
• Isaac N-eebo

U have simplified everything for easy understanding

14th July 2022 at 8:06 pm
• Marcii

I’m a health science major currently taking statistics for health care workers…this is a challenging class…thanks for the simified feedback.

1st April 2022 at 7:57 am
• Emma Carter

That’s good to hear this has helped you. Hopefully you will find some of the other blogs useful too. If you see any topics that are missing from the website, please do let us know!

4th April 2022 at 9:47 am
• Ay

Hello. I liked your presentation, the fact that you ranked them clearly is very helpful to understand for people like me who is a novelist researcher. However, I was expecting to read much more about the Experimental studies. So please direct me if you already have or will one day. Thank you

27th December 2021 at 3:29 am
• Emma Carter

Dear Ay. My sincere apologies for not responding to your comment sooner. You may find it useful to filter the blogs by the topic of ‘Study design and research methods’ – here is a link to that filter: https://s4be.cochrane.org/blog/topic/study-design/ This will cover more detail about experimental studies. Or have a look on our library page for further resources there – you’ll find that on the ‘Resources’ drop down from the home page.

However, if there are specific things you feel you would like to learn about experimental studies, that are missing from the website, it would be great if you could let me know too. Thank you, and best of luck. Emma

24th February 2022 at 2:50 pm