In
epidemiology
Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and Risk factor (epidemiology), determinants of health and disease conditions in a defined population, and application of this knowledge to prevent dise ...
, data or facts about a
population
Population is a set of humans or other organisms in a given region or area. Governments conduct a census to quantify the resident population size within a given jurisdiction. The term is also applied to non-human animals, microorganisms, and pl ...
are called denominator data or population data.
Denominator data are independent of any specific
disease
A disease is a particular abnormal condition that adversely affects the structure or function (biology), function of all or part of an organism and is not immediately due to any external injury. Diseases are often known to be medical condi ...
or condition. This name is given because in
mathematical models of disease, disease-specific data such as the
incidence of disease in a population, the
susceptibility of the population to a specific condition, the disease resistance, etc. disease-specific variables are expressed as their proportion of some attribute of the general population, and hence appear as the
numerator
A fraction (from , "broken") represents a part of a whole or, more generally, any number of equal parts. When spoken in everyday English, a fraction describes how many parts of a certain size there are, for example, one-half, eight-fifths, thre ...
of the fraction or percentage being calculated, general data about the population typically appearing in the
denominator
A fraction (from , "broken") represents a part of a whole or, more generally, any number of equal parts. When spoken in everyday English, a fraction describes how many parts of a certain size there are, for example, one-half, eight-fifths, thre ...
; hence the term "denominator data."
In an
epidemiological compartment model, for example, variables are often scaled to total population. The susceptible fraction of a population is obtained by taking the
ratio
In mathematics, a ratio () shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...
of the number of people susceptible to the total population. Susceptibility to a disease may depend on other factors such as age or sex. Data about a population including age distribution, male/female ratios, and other demographic factors may be relevant as denominator data. Denominator data is not only limited to data describing human populations but also includes information about
wild and
domestic
Domestic may refer to:
In the home
* Anything relating to the human home or family
** A domestic animal, one that has undergone domestication
** A domestic appliance, or home appliance
** A domestic partnership
** Domestic science, sometimes cal ...
animal
Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...
populations.
Value
Denominator data is more important in policy making (e.g., predicting how many nurses will be needed in future decades, or how the prevalence of heart disease will increase as a
population shifts to have more older people and fewer younger ones) than in
clinical trials
Clinical trials are prospective biomedical or behavioral research studies on human subject research, human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel v ...
.
Rates presented without respect to the relevant denominator data can be misleading, and are sometimes derided as ''floating numerators''.
Sources of data
A
census
A census (from Latin ''censere'', 'to assess') is the procedure of systematically acquiring, recording, and calculating population information about the members of a given Statistical population, population, usually displayed in the form of stati ...
is an important source of denominator data.
Other sources of population information include
health insurance
Health insurance or medical insurance (also known as medical aid in South Africa) is a type of insurance that covers the whole or a part of the risk of a person incurring medical expenses. As with other types of insurance, risk is shared among ma ...
programs,
voter registration
In electoral systems, voter registration (or enrollment) is the requirement that a person otherwise Suffrage, eligible to Voting, vote must register (or enroll) on an electoral roll, which is usually a prerequisite for being entitled or permitted ...
records, and
income tax
An income tax is a tax imposed on individuals or entities (taxpayers) in respect of the income or profits earned by them (commonly called taxable income). Income tax generally is computed as the product of a tax rate times the taxable income. Tax ...
records.
Death certificates
A death certificate is either a legal document issued by a medical practitioner which states when a person died, or a document issued by a government civil registration office, that declares the date, location and cause of a person's death, as ...
and other sources of
vital statistics are also sources of denominator data.
In some countries, basic population data has poor quality, so various steps are taken to validate it.
This includes comparing national population estimates against the UN's, checking whether the provided data is internally consistent (e.g., if the reported number of pregnancies is higher or lower than the reported number of births), and if the trends in the population are steady.
Denominator data can also be checked against surveys and external sources of information (e.g., to see whether the number of deaths reported by the government correlates with the number of
coffins sold as reported by the coffin makers); when all sources agree, the data is more likely to be accurate.
See also
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References
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Epidemiology