AB RAJAR'S GUIDE TO RATES, RATIOS AND PROPORTIONS
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This document defines and provides examples of rates, ratios, and proportions in epidemiology. It explains that a rate measures the occurrence of an event in a population over time and includes a numerator, denominator, time specification, and sometimes a multiplier. A ratio expresses the relation between two quantities by dividing one by the other, while a proportion indicates the relation of a part to the whole and has the numerator included in the denominator, often expressed as a percentage.
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AB RAJAR'S GUIDE TO RATES, RATIOS AND PROPORTIONS
- 1. AB RAJAR ASSOCIATE PROFESSOR COMMUNITY HEALTH SCIENCES MUHAMMAD MEDICAL COLLEGE,MPK
- 2. RATEβ¦.. β’ RATE: β’ A rate measures the occurrence of some particular event in a population during a given time period. β’ A rate comprises of following four elements: β’ Numerator. β’ Denominator. β’ Time specification. β’ Multiplier, (E.g. death rate).
- 3. NUMERATOR β’ Numerator refers to number of times an event ( e.g. sickness,birth,episodes of sickness) has occurred in a population, during a specific time period. β’ Note: Numerator is a component of the denominator in calculating a rate, but not in a ratio.
- 4. DENOMINATOR β’ Numerator has little meaning unless it is related to the denominator; therefore epidemiologists has to choose an appropriate denominator while calculating a rate. β’ It may be: β’ Related to the population or β’ Related to the total events.
- 5. DENOMINATOR β’ Related to the population: β’ Mid-year population β’ Population at risk β’ Person-time β’ Person β distance β’ Sub-groups of the population β’ Related to the total events: β’ Eg : number of accidents per 1000 vehicle owners
- 6. Population at risk: β’ Population at risk: β’ Portion of a population that is susceptible to a disease β’ Can be defined on the basis of demographic or environmental factors Prevalence. β’ Population at risk: Examples: β’ Population at risk of developing carcinoma of the cervix: Female population Age > 30 and < 70 years. β’ Population at risk of hepatitis B Those individuals anti-HBc negative Prevalence
- 7. TYPES OF RATEβ¦. β’ DEATH RATE: β’ Death Rate = ππ’ππππ ππ ππππ‘βπ ππ πππ π¦πππ πππ π¦πππ ππππ’πππ‘πππ x 1000. β’ Various categories of rate are: a. Crude Death Rate. b. Specific death rate. c. Adjusted or standardized Rates.
- 8. CRUDE DEATH RATE: β’ CRUDE DEATH RATE: β’ These are actual observed rates such as the birth rate & death rates. β’ The denominator is whole population. β’ Known as Unstandardized rate ( E.g. Crude Birth Rate). β’ CBR= πππ‘ππ ππ ππ πππ£π ππππ‘β ππ‘ π πππππ ππ π π¦πππ πππ‘ππ πππ π¦πππ ππππ’πππ‘πππ ππ π‘βπ π πππ πππππ π¦πππ x 1000. β’ Note: Population of 1st January is less & at 30th December is more, so we take mid-year ( 1st July) population.
- 9. SPECIFIC RATES: β’ These are specific rated due to: β’ Specific causes (e.g. Tuberculosis) or β’ Occurring in specific groups (age or sex groups ) or β’ During specific time period (e.g. annual, monthly or weekly rates). β’ So, in this denominator is a specific segment of population e.g., some specific rates are: β’ IMR β’ MMR β’ Neonatal Mortality rate.
- 10. ADJUSTED OR STANDARDIZED RATES β’ These are obtained by direct and indirect method of standardization or adjustment β age & sex standardized rates.
- 11. COUNT: β’ Count No. of new AIDS cases: β’ City A 58 β’ City B 35
- 12. DIVIDE No. of new AIDS cases No. Year Population City A 58 2004 25,000 City B 35 2004-2005 7,000 β’ City A: 58/25,000/ 1 year β’ City B: 35/7000/ 2 years Divide
- 13. RATIO β’ It expresses a relation in size between two random quantities. β’ Ratio is the result of dividing one quantity by another. β’ It is expressed in the form of x/y or x:y. β’ In ratio, the numerator is not a component of the denominator.
- 14. RATIO Contβ¦. β’ The ratio of white blood cells relative to red blood cells is 1;600,meaning that for each WBC there are 600 RBSs. β’ Others as: β’ Sex-ratio i.e. male to female ratio. β’ Doctor-population ratio. β’ Child-women ratio.
- 15. RATIO. β’ In a class that has 20 male students and 80 female students. β’ We can think about this in several ways. β’ The ratio of boys to girls as 20:80 or 20/80. β’ Aa 1:4 ratio (or 1/4 ratio). [This indicates that for every boys, there are four girls]. β’ From the inverse perspective, i.e., the number of girls relative to the number of boys; in this case the ratio of girls to boys is 80/20 which is equivalent to 4 to 1, i.e., there are four girls for every boys.
- 16. PROPORTION β’ A proportion is a ratio, which indicates the relation in magnitude of a part of the whole. β’ The numerator is always included in the denominator. β’ A proportion is usually expressed as a percentage (%) e.g.: = πππ‘ππ ππ ππ ππππππ ππ‘ π ππππ‘πππ π‘πππ πππ‘ππ ππ’ππππ ππ πβππππππ ππ π‘βππ‘ ππππ ππ‘ π‘βπ π πππ π‘πππ. x 100.
- 17. PROPORTIONS β’ The mortality from bird flue was 44 from total number of infected 123 persons in last year in China. β’ In total, 123 people were infected, and 44 of these died. β’ Therefore, the proportion who died was 44/123, which could be expressed as a decimal fraction (0.36) or as a percentage (36%). β’ This proportion is referred to as the "case-fatality" rate, although strictly speaking, it is a proportion and not a rate.