Scaling rates basically refers to converting very very very small percentages to more digestible, imaginable numbers. Scaling rates is important if you have a very small rate and you want the general public to understand it better. Scaling rates is also important if you want to compare rates from population to population.
Scaling Rates Examples from WHO
The World Health Organization (WHO) has a statistics page from which I will use some 2026 examples.
In India, 89% of births are attended by skilled personnel.
Females in South Africa have a human immunodeficiency virus (HIV) infection rate of 3 per 1,000 uninfected individuals.
Saudi Arabia has 34.06 medical doctors per 10,000 population.
The United States (US) has a suicide rate of 15.6 per 100,000 population.
Notice that each of these rates is expressed with a different denominator. The births attended by skilled personnel is expressed as a percentage – which means “per 100”. The HIV rate is expressed as per 1,000, the medical doctors rate is expressed as per 10,000, and the suicide rate is expressed as per 100,000.
An important, big picture concept behind scaling rates is that the denominator is larger if the rate is smaller. The rate of suicides is a lot smaller than the rate of births attended by skilled personnel. In general, how you determine the denominator has to do with how small the incidence or prevalence rate is to begin with.
Scaling Incidence and Prevalence Rates
As a reminder, an incidence rate is a fraction, where the numerator is the number of those who get the condition during a certain time period, and the denominator is the number of those at risk for the condition in the population at the beginning of the time period. The prevalence rate is also a fraction, but the numerator is those possessing the condition over a certain time period, and the denominator is the number in the population over the same time period. Observe that both of these rates are bound by a time period (such as month or year).
One way to think about why scaling rates is important for public health communication is to imagine number of workers experiencing an injury in a certain month at a business with multiple locations. See below some fabricated numbers for a business with three locations.
Location
Number of workers injured in September 2026
Number working at this location
Location A
1
123
Location B
8
456
Location C
7
789
If we calculate the prevalence of getting injured at each location for this particular month, we need to divide the number who got injured by the number working at the location. If we do that, we get A = 0.0081300813, B = 0.01754385964, and C = 0.00887198986. These numbers are hard to imagine and compare.
Now, let’s scale these rates. Our goal is to make sure that all of these rates are expressed in whole numbers. Both A and C would need to be multiplied by 1,000 in order to be expressed in whole numbers. So if we multiply all of them by 1,000 and use some rounding, we will get the following rates per 1,000: A = 8.1, B = 17.5, and C = 8.9.
Imagine and Compare
Now these numbers are easier to imagine and compare! First, let’s imagine. Some high schools in the US have 1,000 students. Let’s imagine these locations are each the size of one of these high schools. If that were the case, we’d expect between 8 and 9 getting injured in a month at locations A and C, but it would be between 17 and 18 at location B. All of these numbers seem high to me, because you would hope that among 1,000, less than 5 would be getting injured each month.
Next, let’s compare. If we compare, we realize that if we don’t want to get injured, we don’t want to work at Location B. In fact, there is something probably going on at Location B, because it has about twice the injury rate compared to the other locations.
From a public health standpoint, this is the whole point of scaling rates – so we can figure out which ones are too high, and if we want to reduce them, to be able to imagine a realistic target. On the WHO statistics web site, they scale rates so that we can compare countries, and so that they can set reasonable target rates for countries.
Have you ever done a project where you had rates you had to scale? Share your experience in the comments!
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Scaling rates in public health is important if you want to communicate clearly with your audience. My blog post shows you how!
