Female births may be more severely underreported than male births, in countries where women have a lower status. Registration of vital events may be incomplete and selective by sex. Some household surveys or censuses may also have sex bias in reporting of recent live births. It is important that quality of data on female and male births is assessed based on multiple sources providing data on births by sex, number of young children (under age 1, under age 5, or under age 6 in cases where age heaping is suspected) by sex and age, and number of children ever born by sex of the child and age of mother. Also, it is important that data from multiple sources and their quality are analysed over time. In general, it is expected that data quality improves over time with concurrent reduction of sex bias reporting. When this is the case, a substantial increase in the sex ratio at birth, compared to past levels and the normal range, is indicative of some prenatal sex selection.
Misinterpretation of sex ratio at birth may occur when sampling errors are not taken into account when analysing data based on surveys. A ratio of boys to girls of 112, for example, may suggest a prenatal sex-selection when the data are coming from a civil registration system considered complete; in the case of a survey of a small sample, however, it may be well within the confidence interval for the standard number of 107 boys to girls. Therefore it is necessary that countries calculate confidence intervals for survey data on sex ratio at birth.
In some population censuses or surveys, female members of the household may be more likely to be underreported than male members. Recording of household members in population censuses and household surveys may be sex-biased such as when the practice is to record first all the male members before all the female members. In addition, some sex-selective underenumeration may occur in countries or groups of population where women have a lower status. While a common problem in reporting the members of a household is the omission of infants, in some countries girls may be more likely to be underreported than boys.