# Expert opinion on excess death count.

This question was posted the Assessment and Surveillance forum area and has 2 replies.

### Anonymous 29900

student

Normal user

3 Jan 2019, 09:02

Hello experts in the house ,

I was playing with some numbers today and I came up with two different conclusion based on two different approaches that left me wondering which one is the right way to go. This is what the situations are:

I have two time periods t1 and t2 with crude death rate as 0.021 and 0.022 respectively, with a population of 50000 and 53000 at time point t1 and t2.

First approach:

I computed excess death as 0.021*50000 - 0.022*53000 = -116 which means more people died at time t2.

Second approach:

Using the current population at time point 2

(0.021-0.022)*53000 = - 53 that is 53 excess death.

My confusion at this point is how best to compute the excess death ? Scenario 1 or 2?

NB: Any suggestion on good book to read on fundamentals?advanced methods of mortality estimates. I am really interested in this topic more especially in humanitarian settings and would like to learn more.

### Mark Myatt

Consultant Epidemiologist

Frequent user

4 Jan 2019, 16:08

I'll give this a go ...

I think your second method is better than your first method but applying the mortality rates to the populations at t1 and t2 (as you do) will probably overestimate the expected numbers of deaths at t1 and t2 and the difference between them.

I think we need to assume that there are t0, t1, and t2 which are the same distance apart. The first mortality rate (0.021) applies to the time between t0 and t1. The second mortality rate (0.022) applies to the time between t1 and t2.

We should not treat deaths as simultaneous events at which occur only at points t1 and t2. We usually have no information about when each death occurs so we assume (i) deaths can occur at any time point between the two time points, and (ii) deaths occur more or less evenly between two time points. This leads us to applying the rate to the population at the mid-point between the two time points.

Using your numbers ...

We know:

t1 population = 50000 t2 population = 53000

Population growth between t1 and t2 is:

(53000 - 50000) / 50000 = 0.06 (6%)

If we assume constant growth then:

t0 population = 50000 / (1 + 0.06) = 47170

We have:

mid-point population t0:t1 = (47170 + 50000) / 2 = 48585 mid-point population t1:t2 = (50000 + 53000) / 2 = 51500

This gives:

deaths t0:t1 = 0.021 * 48585 = 1020 deaths t1:t2 = 0.022 * 51500 = 1133

The difference is:

d = 1133 - 1020 = 113

I am not sure that I would call these "excess deaths" since mortality of t0:t1 and t1:t2 are almost identical. In this case the additional death are are due to population growth not to changes in mortality.

As for a good book ... this topic is covered in most introductory epidemiology and biostatistics texts.

I hope this is of some use.

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