South Africa 13.9.2018 06:05 am

SAPS corrects wrong population figures used to calculate crime rates

Photo: File Image

Photo: File Image

This had the effect of making the increases in crime rates look smaller and the decreases larger, a UCT researcher says.

After South Africa’s crime statistics for 2017/18 were released, Africa Check quickly discovered that something didn’t quite check out.

UPDATE: The police have now revised the crime ratios following our engagement:

To understand what went wrong, you first need to understand the difference between absolute crime numbers and crime rates.

Absolute numbers refer to the number of crimes reported in a given year. For example, in 2017/18 there were 20 336 murders reported. Crime rates present the numbers of crimes per 100 000 people in a country, for example, 35.6 murders per 100 000 people.

The problem Africa Check discovered is in the crime rate – specifically the way the SA Police Service calculated it for 2017/18.

In 2016/17, the police used population estimates for the end of September each year to calculate crime rates. This is the middle of the financial year, which is the period the crime statistics cover.

SAPS spokesperson Colonel Athlenda Mathe told Africa Check they used Statistics SA’s 2018 mid-year population estimates to put together the 2017/18 crime rates.

There are a number of problems with this.

First, the 2017/18 crime statistics cover the period April 1, 2017 to March 31, 2018. The population estimate SAPS used is not from this period; it’s from June 2018.

Second, by using a later and larger population estimate, it appeared that SA’s population increased from 55 843 011 in 2016/17 to 57 725 600 in 2017/18.

“There is no way the population increased by 3.4% in that year,” Anine Kriegler, researcher at the University of Cape Town, told Africa Check.

Based on Statistics SA’s estimates, the population increased 1.6% between September 2016 and September 2017.

“Police need to be consistent in the population statistics they use.”

The higher population estimate skews changes in the crime rates between 2016/17 and 2017/18.

“It has the effect of making the increases in crime rates look smaller and the decreases look larger,” Kriegler added.

For example, the police’s calculations suggest the murder rate increased by 1.1 percentage points, from 34.1 per 100 000 in 2016/17 to 35.2 in 2017/18. Using the correct population estimate shows the murder rate actually increased by 1.7 percentage points from 33.9 per 100 000 to 35.6.

Decreases in crime rates were amplified by using the higher population estimate.

The police’s statistics suggest assault with intent to do grievous bodily harm decreased 15.6 percentage points from 305.5 per 100 000 people to 289.9. Using the correct population estimate shows the rate is still above 300 and only decreased by 10.8 percentage points.

Report first written by Africa Check

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