Types of Clinical Trials – Based on Study Design

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Types of Clinical Trials – Based on Study Design


This type of study identifies people with a disease (or another variable of interest) and compares them with an appropriate control group which does not have the disease.

An examination is made, comparing the frequency of exposure to this or other factors between the cases and the controls.

It is an analytical observational study which enables the cause-effect relationship to be followed.

If the frequency of exposure or the cause is greater in the group of cases with the disease than in the control group, we can say that there is an association between the cause and effect.

The measurement of the association which quantifies this association is called the “odds ratio“(OR).

In medicine, a case-control study is a cross-sectional type of study which is used to research the etiology of a disease or a given result.

Study in which people with a certain disease or symptom (cases) are compared with others who do not present the disease or symptom under study (controls), with regard to prior exposure to risk factors.

This has been incorrectly called Retrospective Study.

In a case-control study, a single disease but various risk factors or exposures are examined.


Epidemiology adopted this term to refer to the idea of a simultaneous advancement, in time, of a group of individuals defined for possessing a common characteristic or group of characteristics.

The common characteristic is usually exposure to a factor (environmental, pharmacological, occupational, etc).

The term “cohort” is used to designate a group of subjects with a common characteristic or group of characteristics who are monitored over a period of time.


It is an observational, analytical and longitudinal study in which two cohorts differing with regard to the exposure to the factor under study are compared in order to assess a possible cause-effect relationship.

Study in which people subjected to a certain exposure or treatment are compared with people who are not subjected or exposed.


These are studies in which there is a time lapse between the different variables, so that a time sequence can be established between them.

They can be both descriptive and analytical.

In analytical studies, it should be taken into account whether the time sequence is from the cause to the outcome (experimental studies and cohort studies), or from the outcome to the cause (case-control studies).

Any study not focused on an alleged cause-effect relationship, but whose data is used for purely descriptive purposes is considered descriptive.

This type of study is useful for generating etiological hypotheses which should subsequently be contrasted with analytical studies.

Any study which evaluates an alleged cause-effect relationship is considered analytical.

The alleged causal agent may be a factor which is suspected of being able to lead etiologically to a disease or a treatment to prevent or improve a clinical situation.


These are studies in which the data of each subject represents essentially a moment of time.

This data may correspond to the presence, absence or different degrees of a characteristic or disease.

It consists of examining the relationship between different variables in a defined population at a specific moment in time.

These designs do not permit the study of an alleged cause-effect relationship.

Cross-sectional studies are descriptive by definition.

Epidemiological strategy in which observations of numerous factors at the same time are recorded and then a comparison is made between them.

The presence or absence of a disease or other variables (or, if they are quantitative, their level) are determined in each subject.

The analysis of the results can be made in two senses: by comparing all the variables in the individuals who suffer from the disease being studied, comparing them with those who do not suffer from it, or by comparing the prevalence of the disease in different subgroups of the population.

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