Clinical studies can be either observational studies or interventional studies (clinical trials). Observational studies assess outcomes without assigning interventions, while interventional studies compare interventions. Common study designs include cohort studies (longitudinal studies following groups over time), case-control studies (comparing cases and controls), and randomized controlled trials (RCTs; experimental studies randomly assigning interventions). RCTs can be single, double, or triple blind to reduce bias.

1. Study Designs in Clinical
Studies

2. Clinical study
• A research study involving human volunteers (also called participants) that is
intended to add to medical knowledge.
• There are two types of clinical studies: interventional studies (also called clinical
trials) and observational studies.
Ref: https://clinicaltrials.gov/ct2/about-studies/glossary

3. Observational Studies
• A type of clinical study in which participants are identified as belonging to study
groups and are assessed for biomedical or health outcomes.
• Participants may receive diagnostic, therapeutic, or other types of interventions, but
the investigator does not assign participants to a specific interventions/ treatment.
A patient registry is a type of observational study.
( https://clinicaltrials.gov/ct2/about-studies/glossary)

4. Contd..
• The observation can be prospective, retrospective, or current, depending
on the subtype of an observational study.
• Types of observational study models include cohort, case-control, case-
only, case-cross-over, ecologic or community studies, family-based,
and other.

5. Cohort study
• A research study that compares a particular outcome (such as lung cancer) in
groups of individuals who are alike in many ways but differ by a certain
characteristic (for example, female nurses who smoke compared with those
who do not smoke)
• Cohort studies are a type of longitudinal study—an

6. Contd.
• It is an approach that follows research participants over a period of time (often
many years). Specifically, cohort studies recruit and follow participants who share a
common characteristic, such as a particular occupation or demographic similarity.
During the period of follow-up, some of the cohort will be exposed to a specific
risk factor or characteristic; by measuring outcomes over a period of time, it is then
possible to explore the impact of this variable (eg, identifying the link between
smoking and lung cancer in the British Doctors Study.) Cohort studies are,
therefore, of particular value in epidemiology, helping to build an understanding of
what factors increase or decrease the likelihood of developing disease.
(https://ebn.bmj.com/content/22/4/95)

7. Case control study:
• A study that compares two groups of people: those with the disease or
condition under study (cases) and a very similar group of people who do not
have the disease or condition (controls). Researchers study the medical and
lifestyle histories of the people in each group to learn what factors may be
associated with the disease or condition. For example, one group may have
been exposed to a particular substance that the other was not. Also called
retrospective study.
(https://www.cancer.gov/publications/dictionaries/cancer-
terms/def/prospective-cohort-study)

8. Prospective cohort study
• A research study that follows over time groups of individuals who are alike
in many ways but differ by a certain characteristic (for example, female
nurses who smoke and those who do not smoke) and compares them for a
particular outcome (such as lung cancer).
( https://www.cancer.gov/publications/dictionaries/cancer-
terms/def/prospective-cohort-study)

9. Retrospective study:
• A study that compares two groups of people: those with the disease or
condition under study (cases) and a very similar group of people who do not
have the disease or condition (controls). Researchers study the medical and
lifestyle histories of the people in each group to learn what factors may be
associated with the disease or condition. For example, one group may have
been exposed to a particular substance that the other was not. Also called
case-control study.
( https://www.cancer.gov/publications/dictionaries/cancer-
terms/def/prospective-cohort-study)

10. Retrospective Cohort study:
• A research study in which the medical records of groups of individuals who
are alike in many ways but differ by a certain characteristic (for example,
female nurses who smoke and those who do not smoke) are compared for a
particular outcome (such as lung cancer). Also called historic cohort study.(
https://www.cancer.gov/publications/dictionaries/cancer-
terms/def/prospective-cohort-study)

11. Experimental Studies
Randomized Clinical Trials
• A study in which randomization is used to assign patients to treatments. The
purpose of the randomized
controlled trial is to:
(1) to guard against any use of judgment or systematic arrangements leading
to one treatment getting
preferential assignment; i.e., to avoid bias;
(2) to provide a basis for the standard methods of statistical analysis such as
significance tests. (Clinical-Trail-Glossary-Terms.pdf)

12. Double-blind research design
• - A study in which neither the participant nor the researcher knows whether
the participant is in the treatment or control group.
( https://www.fda.gov/patients/clinical-trials-what-patients-need-
know/glossary-terms

13. Double-blind, randomized, controlled
clinical trial
• This is a clinical trial in which the researchers evenly divide study participants
into a group receiving the experimental intervention and a group receiving
standard or no treatment. Neither group knows how it has been assigned.
This practice reduces the chance for a “placebo effect,” in which a treatment
with no active ingredient produces results expected from a treatment with an
active ingredient.
( https://www.fda.gov/patients/clinical-trials-what-patients-need-
know/glossary-terms)

14. Placebo Controlled Study
• A method of investigation in which an inactive substance/treatment (the
placebo) is given to one group of participants, while the test article is given
to another group. The results obtained in the two groups are then compared
to see if the investigational treatment is more effective in treating the
condition.
( https://www.nia.nih.gov/research/dgcg/nia-glossary-clinical-research-terms)

15. Single-blind research design
• A study in which one party, either the investigator or participant, is unaware
of what medication or intervention the participant is taking; also called
single-masked study.
(https://www.fda.gov/patients/clinical-trials-what-patients-need-
know/glossary-terms)