What makes a measure a true rate
In a true rate, every event counted in the numerator comes from the population at risk counted in the denominator. The crude death rate, for example, divides deaths by the midyear population, and every person who died was part of that population. The maternal mortality ratio divides maternal deaths by live births and multiplies by 100,000. The women who died are not themselves live births, so the numerator is not contained in the denominator. Because the people in the numerator are not part of the denominator, the indicator is a ratio, not a true rate.
Why live births are used anyway
The ideal denominator for maternal risk would be the number of women who were pregnant during the year, but that number is hard to count. Many pregnancies end in miscarriage or are never registered. Live births are counted reliably through birth registration, so they serve as an available stand-in for the number of women at risk. This practical choice is the reason the indicator is officially called a ratio. Other indicators that share this structure, such as the infant mortality rate, are often named rates by convention even though they too divide by live births.
| Feature | True rate | Maternal mortality ratio |
|---|
| Numerator | Events from the population at risk | Maternal deaths |
| Denominator | That population at risk | Live births (a stand-in) |
| Numerator inside denominator? | Yes | No |
| Multiplier | Any convenient base | 100,000 |
Why the other explanations are wrong
Counting deaths for one calendar year does not distinguish ratios from rates, since rates are also computed for defined periods. The multiplier, whether 1,000 or 100,000, only scales the result so that it is easier to read; it has no bearing on whether a measure is a rate. Using a subgroup as the denominator does not make a measure a ratio either: specific rates, such as an age-specific death rate, use subgroups and remain rates because the deaths come from that subgroup. Watch out! The large multiplier is a tempting distractor, because maternal deaths are rare and the base looks unusual, but it is a matter of scale only.
Exam takeaway
To classify an indicator, ask whether each person counted in the numerator is also counted in the denominator. If yes, it is a rate; if not, it is a ratio. In the annual report, the nurse labels the maternal indicator as the maternal mortality ratio per 100,000 live births, which allows correct comparison with provincial, national, and international figures.