Understanding observational studies

Last week, we looked at randomised-controlled trials (RCTs), in our series on clinical evidence.

Today we’re going to describe what makes observational studies just as valuable as experimental studies for medical writers.

Although you may regularly source experimental studies for your writing, observational studies are also an important supplement of medical evidence.

Understanding when to use a well-designed observation study can make a difference to your writing.

When used in conjunction with experimental studies, they together can form powerful evidence for your writing.

Why are observational studies useful for medical writers?

Observational studies are different to experimental studies as they solely observe subjects and measure disease variables without assigning treatments.

However, the differences between the experimental and observational study designs can be used as complementary tools.

Research information from observation studies is sourced from natural environments that give context to experimental medical information.

Knowing the meaning behind research help answer medical questions, such as the benefit of a medical therapy for a patient population.

The types of contextual information you can obtain from observation studies are:

• Prevalence
• Incidence and natural history of a medical condition
• Causes or risk factors of medical conditions
• Prognosis
• Associations between the medical condition and exposure to risk factors or medical interventions
• Trends
• Behaviour

What to look for in a high-quality observational study

Well-designed observations studies can provide valid results.

They can look at a diverse population of patients observed in a range of settings, and they can provide useful analytical information.

There are a few key questions you may consider when you’re reading studies to help you select a high-quality design.

• How many patients were lost in the follow-up? Check that it is not significant to cause a difference in the groups for study bias.
• Is the study design appropriate for the research question?
• Is there a distinction between prevalent and incident cases?
• Are the control patients similar to the case patients? (Other than having the exposure).
• Is the sample size adequate and evenly distributed across groups?
• Do estimates appear precise?
• Is causality attributed?
• Have alternative explanations for the outcome been considered and eliminated?
• Are results explained in the context of existing literature?

Types of observational study designs

There are three main types of observational study designs.

The different study designs are based on the research question (hypothesis), sample size and timeline of data collection.

1. Cohort

In cohort studies, all the subjects have exposed to a risk factor (such as cigarette smoke) or a surgical intervention to evaluate the development of an outcome (disease state or medical complication).

Because subjects are selected by their exposure status, cohort studies can be useful studies to assess the benefit of rare exposures.

A single study could also look at multiple outcome variables from a single exposure. For example, the effects of cigarette smoke on lung, cardiovascular and cerebrovascular disease.

Cohort studies can also be useful to learn more about disease progression, staging and natural history of a condition. They are investigated over a period of time and we will discuss what this means later.

2. Cross sectional

Cross-sectional studies look at both exposure and outcome for each participant at a single point in time.

The main difference from cohort or case-controlled studies is some subjects may not be exposed to risks or develop the outcome of interest.

The study design has been described as a ‘snap shot’ of a diverse group of individuals in a range of settings.

Cross-sectional observation studies are useful for understanding the prevalence, causation, treatments, services and other factors associated with outcomes of medical conditions.

3. Case-control

Case control studies look at patients with a known outcome (cases), who are compared to individuals without exposure to known risk factors (control).

They are useful for rare conditions or to assess what makes outcomes different in populations who have the same exposure to risks.

The study design makes it useful to predict whether a specific variable or multiple exposures influences the development of a disease. This makes these study designs useful to calculate odds ratios for disease.

What are retrospective and prospective studies?

You may have heard about the different lengths of time observational studies are conducted.

• Retrospective studies follow up data that has been collected for other purposes and then analysed for the purposes of the study hypothesis. The data collected can occur over a number of years and the studies are often cheaper to investigate.

• Prospective studies are carried out from the present time into the future. They are designed with specific data collection methods, which means prospective studies can be tailored to collect specific exposure data.

However the long follow-up period means it can take a while before outcomes (disease or medical conditions) can be measured.

Do you have any questions or tips for writing about observational studies?