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, 27 (4), 220-7

Confidence Intervals and Sample-Size Calculations for the Sisterhood Method of Estimating Maternal Mortality

  • PMID: 8875734

Confidence Intervals and Sample-Size Calculations for the Sisterhood Method of Estimating Maternal Mortality

J A Hanley et al. Stud Fam Plann.


The sisterhood method is an indirect method of estimating maternal mortality that has, in comparison with conventional direct methods, the dual advantages of ease of use in the field and smaller sample-size requirements. This report describes how to calculate a standard error to quantify the sampling variability for this method. This standard error can be used to construct confidence intervals and statistical tests and to plan the size of a sample survey that employs the sisterhood method. Statistical assumptions are discussed, particularly in relation to the effective sample size and to effects of extrabinomial variation. In a worked example of data from urban Pakistan, a maternal mortality ratio of 153 (95 percent confidence interval between 96 and 212) deaths per 100,000 live births is estimated.

PIP: The sisterhood method of calculating maternal mortality, which relies on surviving sisters to report on sibling mortality, is easy to use in the field and provides fast calculations of maternal mortality in settings with scarce data on vital statistics. This report describes a method of calculating sampling error for the sisterhood method. Calculation of sampling error allows investigators not only to report results with interval estimates but also to project sampling variability associated with various sizes of sample surveys in order to determine the most desirable sample size for estimation of maternal mortality rates. After explaining the statistical methodology, data from Karachi, Pakistan were applied to the model. Tables illustrate 1) the numbers of reported sister deaths in relation to maternal mortality rate, total fertility rate, and sample size for studies in nine countries and 2) the approximate sample sizes required to achieve the desired margin of error. The method to compute sample sizes necessary to enable a comparison of two maternal mortality rates based on sisterhood surveys is also presented, and the appropriateness of using the binomial formula is discussed. The problem of lag time in sisterhood surveys (where the time reference period may range from 5.7 to 15 years) is considered, and it is noted that use of the sisterhood method may be inappropriate in settings where maternal mortality has been changing rapidly. It is concluded that efforts should be made to reduce other potential sources of bias in retrospective, cross-sectional studies. The appendices contain the method to derive the standard error of an estimate of the maternal mortality rate and the method for calculating the total fertility rate and its sampling uncertainty.

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