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Different types of clinical trials available depend on the purpose of the study. Clinical research involving the human subjects known as clinical trials. It does not necessarily mean that you have to give medication/ intervention to the research subjects. Some studies does not given any study medication, even some studies only observe healthy volunteers for particular period of time to achieve their objectives. Depend upon the purpose of study different types of clinical trials conducted.

Here one thing should be noted that types of clinical trials are different from types study designs used in the clinical studies.  Depend upon the purpose, types of clinical trials broadly divided as follows.

1. Interventional trials:

Intervention trials also known as Experimental studies. In this type, one or more treatments or interventions allotted to a group of research subjects to measure the treatment related outcome. Depend upon the allocation of intervention to the study groups, again based study designs, interventional trials divided into following types.

a. Randomised control trials (RCT):

Interventions allotted to the research subjects based on randomization method and pattern used for the study. Intentional allocation of treatment to particular subject is not possible in this study design, it helps to avoid bias. These are the  most commonly performed interventional trials. RCT can be divided into two types

i. Parallel randomized controlledtrials: Treatment groups receive the randomly allotted treatments till the end of the study.

For e.g.: If intervention ‘X’ allotted to group A, and intervention ‘Y’ to group B. Group ‘A’ and ‘B’ will only receive intervention ‘X’ and ‘Y’ respectively till end of the study

ii. Cross-over randomized controlledtrials:   Treatment groups will receive both the interventions in the study period at any phase.

For e.g.: In the same case mentioned above, treatment group A and B will receive both the interventions X and Y. 

b. Non randomised control trials:

In this design, you can allot particular intervention to particular group.

c. Pre-post trials:

Here, usually one group used for study. One group of subjects observed for a particular period of time with no intervention. After some time the intervention allotted. Outcome compared with pre and post intervention to know the effect of intervention.

Note: Types of clinical trials are different from phases of clinical trials that is phase I,II,III, and IV. All phases of clinical trials except phase IV trials are a type of interventional trials. You can also see the overview of phases of clinical trials here.

2. Observational studies:

These studies also known as Non-interventional studies or epidemiological studies. It means interventions are not allotted to research subjects. It mean that the patients may or may not taking their regular medication but these medications are not allotted by the researcher like a interventional study.  Different types of Observational studies based on their study design are as follows.

a. Cross sectional studies:

Cross sectional studies also called as prevalence studies. These studies conducted at one point of time. Study duration also very less compared to other observational studies.  The main objective of these studies to find a possible link between the risk factor or cause and interested outcome. Usually, these studies done retrospectively.

For e.g.: A study finding that how many persons in a group are chronic alcohol abusers and how many persons have liver cirrhosis in that group.

b. Cohort studies:

Cohort is nothing but a group of people with similar status or characteristics. It involves several types of cohorts or groups. These cohorts observed over a period of time.

For e.g. several groups observed for a particular period to know which group developed interested outcome which group not developed the outcome.

 These studies done usually based on the outcome from a cross sectional studies. These studies can be done prospectively as well as retrospectively. Prospective studies are of great value compared to retrospective studies because retrospective studies may have recall bias.

c. Case-control studies:

Researcher recruits the subjects based on their disease status. One group of subjects having a disease act as a test group and another group of subjects not having a disease act as control. In both groups, researcher look back at various factors of those subjects. If a particular factor present in all the diseased subjects, that factor will be compared with control subjects to draw the exposure-outcome relationship conclusions.  These studies are retrospective studies and have recall bias.

For e.g: A case control study to compare the kidney failures in diabetic patients as compared with non diabetics.

Apart from these studies, other types of observational studies are also available like ecological studies and proportional mortality studies etc.

Other clinical trials:

Apart from interventional and observation clinicaltrials, other types are also available. They are prevention trials and screening trials etc.

a. Prevention trials:

These studies conducted using healthy volunteers to find the prevention strategies. These studies conducted for a long period of time.

b. Screening trials:

These studies conducted in a general population or in a subjects with high risk to a particular disease. The objective of these trials is to identify diseased subjects as early as possible before showing disease symptoms.

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