Cohort , from Latin cohors, is a term with two different meanings. On the one hand, it is a tactical unit of the Roman army which, throughout history, had different conformations. On the other hand, cohort is a series, a set or a number .

A roman cohort , therefore, it was an army unit that usually consisted of only one type of soldier. A legion it was formed with ten cohorts, which were numbered from I to the X . The cohorts, in turn, were made up of three manipules (formed by two centuries ). The cohort I On the other hand, it had exceptional characteristics and consisted of five double centuries.

There were, outside the army, civil security cohorts, such as urban cohorts (who were in charge of daytime security) and vigilum cohorts (dedicated to guarantee night security).

As a set or series, the notion of cohort is used in the demography , the epidemiology and the education . A cohort is a group of people who share the same event within a certain time period.

This means that a cohort, for example, can be made up of all people born in a city X between 1970 and 1972 . Another example of a cohort is the group of students who begin their primary studies in the same year and, therefore, should conclude them together. In this sense, cohort is synonymous with promotion.

For the biology On the other hand, a cohort is also a group of individuals, in this case related to a taxonomic superorder.

Cohort study

The objective of a cohort study (which is considered analytical, observational epidemiological and prospective longitudinal) is to compare the frequency of a disease or of a particular phenomenon between two groups of people, provided only one of them is exposed to a certain risk factor. In order to select the individuals that are part of the study, the presence of certain characteristics and exposure to that factor is taken into account.

Some of the fundamental points of a cohort study are the following:

* identify and select individuals healthy within a population (this group is called sample);
* check if the risk factor is present in the sample, measuring the exposure variables;
* track the cohort;
* Measure the outcome variables to verify the presence of the disease in question.

Among the advantages of a cohort study, it is correct to say that it allows:

* study factors uncommon exposure;
* analyze the different effects that may arise from such exposure;
* whenever it is considered from the beginning of the study, simultaneously observe the effects of more than one exposure;
* estimate the incidence (the number of new cases of a disorder in a given population and at a certain time) and the Relative risk (the ratio between the risk of the group with the exposure factor and that of the one without it);
* clearly define the sequence of events of interest, such as exposure-disease;
* avoid making a perception error known as survival bias, which is characterized by ignoring the results that do not exceed a certain process;
* select subjects more efficiently;
* effectively control the measures.

On the other hand, the following disadvantages are also noted:

* normally demands samples of a considerable number;
* it does not offer great efficiency when the latency periods are very long;
* it usually requires a significant investment of time and money;
* It is not recommended for the analysis of rare diseases or events.