Changes in the number of

reported crimes and changes

in the crime rate don’t reconcile

On Thursday Police Minister Nathi Mthethwa presented

the SAPS 2012/2013 national crime statistics. In his

statement he listed the increases or decreases of various

categories of crime in terms, not in absolute numbers,

but in the rate or ratio per 100 000 of the population.

Thus he stated, for instance, that total sexual offences

decreased by 0.4% and robbery in non-residential

premises by 0.6%. He added that there had been an

increase of 1.2% in aggravated robbery, 3.3% in

residential burglary, 0.6% in commercial crime, and 3.6%

in residential robberies.Mthethwa also stated that "we

have witnessed a slight increase of 0.6%" in the murder

rate.

In absolute terms however there were 16 259 murders in

2012/2013, 620 up from the 15 609 recorded in

2011/2012. This is a 4.2% increase. How then can the

differential between a 0.6% increase in the murder rate

be reconciled with this 4.2% increase in the total number

of murders?

The short answer is, it can’t.

One possible explanation is that the population of South

Africa grew by 3.3% between 2011 and 2012. However,

Statistics South Africa estimates that the population of

South Africa only grew by 1.34% between 2011 and 2012

(see here.)

In fact, what seems to have happened is the following:

The 2011/2012 crime statistics’ ratios per 100,000 of the

population were calculated using Stats SA’s 2011 mid-

year population estimate of 50 586 757. For the

2012/2013 crime statistics the SAPS worked out the ratios

using the Stats SA estimate of a population of 52 274 945

in 2012. It then worked out the change in the ratios per

100 000 of the population between the two.

However, the differential between the 2011 StatsSA

estimate used by the SAPS and the 2012 one is 1 688 188,

an implausibly large 3.34% gap. This doesn’t reflect a

real change in the population, but rather an adjustment

upwards in StatsSA’s estimate of the population

(presumably post-Census 2011). According to these

adjusted estimates the population in 2011 was 51 579 548

– only 695 397 (or 1.35%) fewer than in 2012.

If one uses the updated population figures of StatsSA for

2012 and 2011 then the murder rate actually increased by

2.78% (not 0.6%). There was a 1.53% increase in the rate

of sexual offences (not a decrease of 0.4%) and an

increase of 1.30% in non-residential robberies (not a

decrease of 0.6%). The rate of aggravated robbery was up

3.24% (not 1.2%), that of residential burglary by 5.33%

(not 3.3%), commercial crime by 2.61% (not 0.6%), and

residential robbery by 5.65% (not 3.6%).

The table below sets out the change in the number of

crimes committed, in various categories, between

2011/12 and 2012/2013 – and then the change in the ratio

per 100 000 of the population using the old Stats SA

population estimates for 2011 versus the new ones.

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