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 Chapter 6:  Population Health Assessment




Mass field-surveys of entire animal populations by immunodiagnostic and other mass diagnostic tests is central to gathering population health status. Two approaches are considered here:

a) Surveillance procedure that emphasize the case or infected herd finding procedure, and,

b) Surveys.

Surveys can be thought of as being either direct or indirect. In a direct survey the variable of interest is measured directly on the sample unit, e.g. antibody titers in a group of animals; in an indirect survey the observations are made by a person other than the investigator and are reported on a survey form or questionnaire, i.e. the investigator has only indirect access to the variable being measured in the sample. Since he/she is the one to make inferences from these observations to the entire population, it is important that one devise an appropriate tool to gather reliable data.

It is vital to consider the following then:

a) Population health assessment approaches

- Sampling and Inference

- Surveys

- Surveillance

b) Population health assessment tools

- Screening test systems

- Questionnaire methods

Sampling and Inference

Assignment: pp. 32-34, 234-244 Text

Surveys are generally done to discover the frequency and distribution of diseases (descriptive surveys), and to attempt to explain such distributions (explanatory surveys). In veterinary medicine surveys are most often aimed at prevalence of disease or prevalence of infection.

Other aims include assessing exposure to a factor, obtaining opinions about say the efficacy of a drug, measuring mortality, etc.

Surveys are usually initiated on request by a policy maker, a research advisor, a government official, or to follow up an interesting observation by an astute veterinarian, veterinary student, or a concerned client.

Surveys scan be classified into various types according to their objectives as listed below. Some of these types of surveys are more important than others in epidemiology and will be elaborated on.

Types of Surveys


Descriptive survey to determine the extent of a problem and to quantify it
Explanatory survey

to find determinants

Cross-sectional survey

to measure prevalence of a variable

Longitudinal survey to measure incidence of a variable
Single purpose survey to evaluate one variable
Census survey to account for the total population
Sample survey to account for the total population by interference from a sample of that population.

Combinations of these types of surveys are of course possible. The most common type of survey is probably the sample survey. An important requirement for such surveys is that the sample represents the population from which it is drawn as accurately as possible. This may be achieved by various methods of probability sampling.

The following is a summary discussion of sampling and inference; for a more detailed presentation the student may refer to a number of statistical texts.


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