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Just for the clarity we want to add the following description of Population at risk from WHO publication- "Basic Epidemiology" by Bonita et al (1):
Population at risk
An important factor in calculating measures of disease frequency is the correct estimate of the numbers of people under study. Ideally these numbers should only include people who are potentially susceptible to the diseases being studied. For instance, men should not be included when calculating the frequency of cervical cancer.
The people who are susceptible to a given disease are called the population at risk, and can be defined by demographic, geographic or environmental factors. For instance, occupational injuries occur only among working people, so the population at risk is the workforce; in some countries brucellosis occurs only among people handling infected animals, so the population at risk consists of those working on
farms and in slaughterhouses.
References:
1. Basic epidemiology / R. Bonita, R. Beaglehole, T. Kjellström. 2nd edition. World Health Organization 2006. Geneva.
I thank Dr Philip Sedgwick for such a lucid and simplified account of Population at Risk. Here I would like to add that if a cohort study takes place in the hospital (because of certain constraints, like not able to go in the community due to lack of funds) selection bias occurs unless all the cases from the community are going to that particular hospital and are representative of the community. In such an event we cannot measure the population at risk including those e.g., who were practicing high risk behavior (Case cohort)and not doing so (control-cohort).
Defining the population at risk
Just for the clarity we want to add the following description of Population at risk from WHO publication- "Basic Epidemiology" by Bonita et al (1):
Population at risk
An important factor in calculating measures of disease frequency is the correct estimate of the numbers of people under study. Ideally these numbers should only include people who are potentially susceptible to the diseases being studied. For instance, men should not be included when calculating the frequency of cervical cancer.
The people who are susceptible to a given disease are called the population at risk, and can be defined by demographic, geographic or environmental factors. For instance, occupational injuries occur only among working people, so the population at risk is the workforce; in some countries brucellosis occurs only among people handling infected animals, so the population at risk consists of those working on
farms and in slaughterhouses.
References:
1. Basic epidemiology / R. Bonita, R. Beaglehole, T. Kjellström. 2nd edition. World Health Organization 2006. Geneva.
Competing interests: No competing interests