A mortality rate (MR) expresses the number of events (deaths) that occur in a population of known size that is at risk for the death event during a specific period of time (usually called the recall period).
Mortality rate = Total deaths during period / (mid-period population at risk x duration of period)
The mid-period (or midpoint) population is estimated by (i) taking the population at the end of the period; (ii) adding one half of the deaths during the period; and (iii) subtracting one half of the births during the period.
- duration of period: 120 days
- births during period: 360
- population at end: 18,300
- deaths during this period: 445
Mid-period population at risk: 18,300 + 0.5(445) – 0.5(360) = 18,343
Unit of expression: per 10,000 people per day
MR = [445 / (18,343 x 120 days) ] x 10,000 = 2.02 deaths per 10,000 people
Interpretation. In this population, on average about 2 people out of 10,000 died every day during the 120-day period analyzed.
Checchi, L Roberts. Interpreting and using mortality data in humanitarian emergencies. Network Papers. Humanitarian Practice Network.No.52, Sep 2005. Internet. Accessed on October 9, 2012.