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P/F Ratios

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P/F Ratios: BACKGROUND

Data on births are often needed to calculate the denominator for the maternal mortality ratio. The Brass P/F Ratio Method (Brass, 1968) is used to evaluate the completeness of birth recording for a given reference period preceding the census.

Most censuses and many household surveys include fertility questions on the number of children ever born to women and whether they had a birth in the year preceding the survey. The P/F (Parity/Fertility) ratio technique developed by Brass calculated fertility indirectly by using the average number of children ever born to women in 5-year age groups and age specific fertility derived from births in the year preceding the survey. The Arriaga technique (Arriaga, 1983) is similar to P/F ratios but links data for more than one date. The P/F ratio method assumes constant fertility in the past while the Arriaga method does not.

P/F ratios reflect the consistency between information on lifetime fertility and current fertility across women’s age groups. Lifetime fertility or average parity (P) is considered to be accurate among younger compared to older women because they have fewer recall errors and omissions. By contrast births in a given reference period can be distorted due to date displacement, which is likely to occur to the same extent across age groups. By cumulating recent births, average parities can be calculated but these parity equivalents (F) will be smaller than reported average parities if recent births are not completely recorded,. The degree of underreporting can be evaluated by analyzing P/F ratios by age. If constant fertility is assumed, the average of the ratios for ages 20-24 and 25-29 indicates the consistency of birth information.

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A general handbook on fertility and mortality data:

United Nations
United Nations

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US Census Bureau


Population Studies

For examples of how to calculate P/F ratios for evaluating census data:

Measure Evaulation

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East-West Center

United Nations

United Nations

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