South Africa’s annual crime statistics provide useful information, but a calculation error in this year’s stats has skewed crime rates – and how much they have increased.

This is according to fact-checking website Africa Check, which noted that the SAPS had used Statistics South Africa’s 2018 mid-year population estimates to put together the 2017/18 crime rates.

There are a number of problems with this, Africa Check said.

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

“Second, by using a later and larger population estimate, it appeared that South Africa’s population increased from 55,843,011 people in 2016/17 to 57,725,600 in 2017/18 – a difference of more than 1.8 million people,” it said.

Anine Kriegler, researcher and doctoral candidate at the Centre of Criminology at the University of Cape Town, told Africa Check that it was almost impossible that the national population increased by 3.4% in that year.

The higher population estimate may seem like a small issue, but it 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 explained.

For example, the police’s calculations suggest that the murder rate increased by 1.1 from 34.1 per 100,000 in 2016/17 to 35.2 in 2017/18. Using the correct population estimate shows that the murder rate actually increased by 1.7 from 34.1 per 100,000 to 35.8.

Kriegler said this increase was ‘a lot larger and a lot more worrying’.

“This is not the first time the police have been criticised for using incorrect population estimates to calculate crime rates,” Africa Check said.

“Going forward, the service must commit to using correct and consistent population estimates – and they need to be transparent about it”.

The SAPS responded by admitting that it did use the incorrect population figures, but stressed that it did not change the actual crime stats (number of crimes) recorded, only the rates.

Read: South Africa crime stats 2018: everything you need to know