Medical research shapes healthcare every day, but not all studies are created equal. Knowing the differences can help you feel more confident reading health information online and understanding what it might mean for your own health.

Different types of medical studies are used for different purposes and can answer different types of questions. They are generally classified as either experimental or observational.

  • Experimental studies: One group is given a particular drug or treatment and compared with a group that does not have the treatment. Experimental studies may show causation, depending on how they are carried out.
  • Observational studies: Researchers look at what happens over time but do not change anything or give any treatments. Observational studies cannot show cause-and-effect.

Let’s look into the different types of study in more detail.

Clinical trials are a particular type of experimental medical study done only after preclinical research (nonhuman studies) is carried out. Adults and children alike are able to participate in clinical trials, and you may or may not have a health condition.

These studies test how a drug or other medical treatment works. Other medical treatments include surgical procedures, behavioural interventions, and medical devices.

There are five clinical trial phases:

  • Phase 0: Participants take an extremely low dose of the treatment, usually for a week or less, to check for any side effects.
  • Phase 1: A small group of people (20 to 80) have a treatment, and researchers look for side effects and establish a safe dose.
  • Phase 2: Once a treatment is found to be safe in phase 1 trials, it is then tested in a larger group of people, around several hundred participants.
  • Phase 3: These studies involve larger populations of around 3,000 people, including those in different regions and countries. Randomized controlled trials are often used in this phase.
  • Phase 4: After treatment approval, testing is not over. Continued trials monitor the effect of the treatment in a wider population over a longer timeframe to give more data on how it is working.

The smaller, earlier clinical trial phases are generally treated with caution, as more evidence is needed to be sure of a particular effect. As more studies are done, more about how a treatment may work in the wider population becomes known.

If you’re interested in getting involved in clinical trials, speak with your doctor. They’ll be able to let you know if you’d be suitable to take part in any.

RCTs are generally considered the “gold standard” of medical studies as they are the most likely of all studies to truly show cause-and-effect. These are often the type of study used for phase III clinical trials.

The “controlled” part of RCT refers to treating one group while the other group receives a “control.” This control may be a placebo given in the same format as the intervention, or it may just be the typical treatment that already exists for that condition.

The “randomization” part of RCTs means participants are randomly assigned to the treatment or a nontreatment group, and neither the researcher nor the participant gets to choose.

However, if having no treatment would put you at risk, you will not be assigned to this group. Along the same lines, if a treatment starts to show benefits in the treated group, the trial is stopped so that everyone can receive the benefits of the treatment.

RCTs may be “single-blind” or “double-blind.” In single-blind studies, researchers know who is in each group, but participants don’t. In double-blind studies, neither the researchers nor the participants know. (However, it’s always possible to find out if needed.) The benefit of blinding is that it reduces potential bias.

Non-randomized trials

In non-randomized trials, researchers can place participants in a particular group, or participants can choose themselves.

The benefit of this is that it reflects what happens in everyday life. This type of trial is also less time consuming and costs less. However, this does mean there is a higher chance of bias, which can affect the results of a study.

Observational studies are exactly what they sound like: watching or observing, without any intervention from the researchers.

These types of studies are often used in epidemiological research.

You cannot determine cause-and-effect from an observational study. However, you may be able to find potential correlations — or links — that may be used to inform experimental studies going forward. Observational studies are also useful when experimental studies cannot be carried out, for example, for ethical reasons.

Next, learn the four main types of observational study:

Case study

Case reports are individual reports of a condition in a person and are usually a description of the findings. They are considered the starting point of medical research and can bring attention to a rare disease or a new finding.

The findings may not apply to larger populations since they only focus on one or a few individuals.

Cohort study

These studies involve following a group, or cohort, of people with or without a particular factor over time, often many years, to see if they develop a condition.

This could be looking forward (prospective), e.g., following them for 10 years, or looking back (retrospective), e.g., what happened over the past 10 years.

Case-control study

These compare a group of people with a disease to a group without it to identify possible causes or risk factors. Since the disease has already developed, these studies only look backward to see what might have contributed in the past.

The benefit of this type of study is that the effect has already taken place, and you just need to go back and collect the data. The drawback is that it often relies on people remembering things in the past, so there’s a potential for error, especially if you’re looking back over a long period of time.

Cross-sectional study

These take a “snapshot” of data from a population at a single point in time and can give critical information, for example, how common a particular condition is.

A common way to collect this data is with a survey.

The con, however, is that you cannot find out anything about the cause of a condition, nor what the best treatment might be.

Preclinical, or nonclinical research, is done before any clinical trials in humans begin. These studies are usually experimental rather than observational.

Nonclinical research may be in vitro or in vivo.

  • In vivo means in a living organism (human and animal studies are all in vivo).
  • In vitro means outside a living thing, e.g., in a test tube or in cells outside a living organism.

Nonclinical research does not always translate to humans, so it’s important to understand its place in the research timeline. These early studies can help inform future research, but are not considered strong evidence on their own.

Medical studies may be either primary or secondary sources.

The original papers written by the researchers who conducted the study are primary research.

Secondary research analyzes, interprets, or summarizes primary research. This type of research does not present new data itself but instead synthesizes and evaluates primary research findings.

Secondary sources include:

  • Review articles: These summarize findings of multiple studies.
  • Systematic reviews: These summarize the findings of multiple studies using a particular method or protocol. They are more rigorous than standard review articles.
  • Meta-analyses: These provide even stronger evidence as they use statistical analysis to review findings of multiple studies.

Other considerations

When interpreting medical studies, you’ll also want to take into account certain other factors.

For example, researchers often note limitations of the research in the discussion section. It’s important to understand any limitations they mention or that you notice, as it can affect the strength of the evidence.

Other factors to keep in mind include:

  • Study age: Older research may have been disproved or built upon, so generally, studies within the past 3 to 5 years will give you more accurate information. However, certain medical findings haven’t changed. Reading around a subject can help determine where a particular medical study fits into the overall story.
  • Sample size: Usually, smaller studies are less generalizable to the wider population.
  • Demographic limitations: If the study only involved certain groups of people, its findings may not always apply to other groups.
  • Preregistration: In the United States, preregistration for clinical trials is mandatory. This means that the research plan must be documented in advance and cannot be changed once the trial has started.
  • Conflicts of interest: Authors must declare conflicts of interest in their research. For example, if they are funded by the company whose product they are testing, or are themselves that company. Medical papers have a section stating this. Although it does not mean results are biased, it’s important that this information is clearly mentioned and can be considered by people reading the research.
  • Peer review: This means the medical study has been checked by other medical professionals before publication. Not all studies are peer-reviewed.

Observational studies watch what happens without intervention, while experimental studies (like clinical trials) test specific interventions.

Observational studies reveal associations but cannot show cause. Experimental studies can help show cause-and-effect.

Randomized controlled trials minimize bias through random assignment and often use blinding to reduce bias. This helps ensure that differences in outcomes are due to the intervention, not other factors.

It means results are unlikely to have occurred by chance (often p < 0.05). However, statistical significance does not always mean results are important for real-world health decisions.

A simple example might be: if a drug is in a format that people may have difficulty taking or it costs a lot, then the real-world impact, or “clinical significance,” will be lower, as people will not realistically take the drug.

You’ll want to look at different factors including research design, sample size, peer review, clear disclosure of conflicts of interest, and whether the findings are consistent with other studies.

Also consider if the study population is similar to the group you’re interested in.

Correlation means two things are related, but it doesn’t necessarily mean that one causes the other. Many other reasons may account for the link between the two.

Causation means one causes the other. Sometimes a correlation can indicate causation, but not always. Randomized controlled trials are more likely than other study types to show cause-and-effect.

Animal studies allow researchers to test safety and basic mechanisms before trying the drug or medical intervention in humans.

The results from an animal study do not always translate to humans.

A meta-analysis combines results from several studies and uses statistics to identify overall trends. This means they usually provide strong evidence compared with other study types.

Larger sample sizes usually mean more reliable and generalizable results. However, the study quality and design are also important to take into account.

In vitro studies are done in test tubes or petri dishes (outside the body).

In vivo studies are done in living organisms (like animals or humans).

Be cautious about applying findings to other groups of people not looked at in the study. Results may not always generalize to different ages, sexes, or races.

Healthline has a strict editorial process. When citing medical studies in our articles, we ensure only to use peer-reviewed scientific journals.

Though Healthline is not a medical journal, and so is not peer-reviewed in the same way journals are, all our articles go through a rigorous medical review process to help ensure scientific accuracy.

Medical professionals regularly review and update articles to reflect current knowledge and uphold content integrity. We also welcome feedback from our readers, and each Healthline article includes a feedback widget for you to share your views.

Understanding medical research can help you understand more about a particular health condition and how current research could potentially apply to you.

However, not all medical studies offer the same strength of evidence. Different study types have particular strengths and limitations. Understanding these differences can help you determine what a particular study might be able to tell you, and what it can’t.

Randomized controlled trials and meta-analyses are generally considered the strongest evidence, while early laboratory and animal studies are less directly applicable to humans.

Knowing how to spot bias, limitations, and the difference between correlation and causation can help you better interpret health news and research accurately. Being an informed health consumer can also help you in your healthcare journey.

However, it’s important to always speak with a healthcare professional about your personal health decisions. Their medical training, combined with knowledge of your personal health history, means they can offer the best advice for your specific situation.