No. Mortality rates may also differ between different time periods. In most countries mortality rates have fallen over the past years. With the AIDS epidemic in Southern Africa, infants and under-5 mortality rates are rising.
Therefore, mortality rates allow you to compare different groups of children, either in different places (districts, regions, provinces or countries) at the same time or in the same place at different times (months or years).
Mortality rates in the same place often vary over time.