RAMOS (Reproductive Age Mortality Surveys) studies seek to identify all female deaths in the reproductive period, using a combination of approaches, such as cross-sectional household surveys, continuous population surveillance, hospital and health center records, and key informants (WHO, 1987).
Direct Estimation relies on asking questions about maternal deaths in a household during a recent interval of time, say one to two years. These questions can be asked in the context of a household survey or a census of all households, although as yet experience with the latter is fairly limited (Campbell, 1999).
Both these types of methods provide up-to-date estimates but are time-consuming and costly because they require large sample sizes to obtain single-point estimates with sufficiently narrow confidence intervals to enable monitoring of time trends.
The sisterhood method goes some way to overcoming large sample size requirements by interviewing adult respondents about the survival of all their sisters. The indirect method (Graham, Brass, and Snow, 1989) involves fewer questions to respondents but provides a pooled estimate that relates statistically to a point around 10-12 years prior to the survey. The direct method (Stanton, Abderrahim, and Hill, 2000) provides a more current estimate at about 3-4 years prior to the survey, but requires more questions and is more costly and time consuming.
Maternal mortality ratios are only a broad indication of the level of maternal mortality, rather than a precise measure, because of the limitations inherent in most measurement methods.