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Friday, September 20, 2013

Did the SAPS cock-up their crime stats?


The Ratcatcher - politicsweb

20 September 2013

Changes in the number of reported crimes and changes in the crime rate don't reconcile

Did the SAPS cock-up their crime stats?

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%).

Politicsweb has published a table that 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.

Source: www.politicsweb.co.za

Related Post: 1.7 million crimes a year not reported – SAIRR

3 comments :

PayBack said... .....Click here to refresh this blog

Yes, they really believe the old saying: "Bullshit baffles brains", only it doesn't work with intelligent people!

Tia Mysoa said... .....Click here to refresh this blog

An extract from a statement issued by The Institute for Security Studies, September 20 2013:

In order to present accurate changes in the crime rates, the SAPS should have recalculated the crime rates for 2011/12 (as well as their five year and eight year comparisons) using the revised 2011 population figures. This would have yielded an accurate comparison between the 2012/13 crime rates and the 2012/11 crime rates.

Why is it a problem?

The problem is that this approach is statistically incorrect, because the changes in the crime rates are calculated on inaccurate population estimates.

Consequently, changes in the crime rates that were presented are inaccurate and therefore misleading. For example the change in the murder rate was given as a 0,6% increase. However, when using the correct population estimates for 2011/12 and 2012/13, the murder rate actually increased by 2,8%. This error does not change total numbers of 16 259 murders that were recorded in 2012/13.

While this slight change may not seem like much, it can lead to inaccurate conclusions. For example, the Minister of Police is under the impression that the rate of sexual offences had decreased by 0,4% in 2013/12 compared with 2011/12. However, using the correct population figures for 2011/13 the rate of sexual offences actually increased by 1,5%. Similarly, the figures provided by the SAPS to the Minister caused him to erroneously announce that non-residential (or business robberies) had decreased by 0,6%, when in fact it had increased by 1,3%.

Presenting crime statistics in this manner is an internationally acceptable standard, as it allows for an accurate measure of how crime rates that specifically impact on individuals (e.g. murder, assault), compare across different localities with different population sizes. It also allows for an accurate trend assessment due to the changes in the size of the national population each year. However, it is only correct when consistently accurate population estimates are used for each year.

Read full report here.

Anonymous said... .....Click here to refresh this blog

Of course they did...
Arkan.

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