Friday, 1 April 2011

Lap dancing and rape in Camden: part 2

Read all the posts in this
[1] [2] [3] [4] [5]

Let's look in depth at one year's change in rape statistics in Camden. In 1999, the Metropolitan police recorded 72 reports of rape. In 2000, the number was 88. The Met numbers are available to the public so can’t be disputed. And those numbers went up. This much the Lilith report got right. But is that all there is to the story?

The problem with numbers on their own is they don't say anything about context. The number may rise from year to year, but if the population is going up as well, the rate might not be changing at all.

Imagine, for instance, if a paper claimed London has 1000% more Chinese restaurants than it did 40 years ago, but didn't report the relative populations for those years. You wouldn't think much of the numbers. Of course the raw number would have gone up - the population got a lot bigger from 1970 to 2010. Without context, the numbers don't mean very much.

When the population grows, you have to take that change into account. What you need is not just the raw number of crimes reported, but also the population of the area from one year to the next. This is used to calculate not the number of crimes, but the rate. Rate and number are two different things, but many people (even those who should know better) use them interchangeably, and this creates confusion.

You don't have to be a London native (or even a Daily Mail reader) to know the population is going up. It's on the rise in Camden. But is it going up enough to make the rate of rapes look different from the number? Let's see.

Whenever numbers of incidents are reported, they should be used to calculate the rate of occurrence. This gives you an estimate of how many times the crime occurred per 100,000 population. So let’s look at those rape numbers again. For the year 1999, we have 72 rapes reported in Camden and - according to National Statistics - a population of 195,700 people.

To determine how many rapes occurred per 100,000 residents, we divide the number of rapes by the total population. Then we multiply by 100,000:

72 ÷ 195,700 × 100,000 = 36.8

This tells us that in 1999, there were 36.8 reported rapes for every 100,000 residents of Camden. Performing the same rate calculation for 2000, when the population was 202,800 and the number of rapes 88, gives us a rate of 43.4.

Mathematically calculating the change in rate from one year to the next gives us the percent change, be it a rise or a fall. The change in rate from 1999 to 2000, or the change from 36.8 to 43.4, is 17.9%.

That is considerably different from 50%. So the rate (which is what counts) of rapes in Camden did not go up by 50% after the lap-dancing clubs opened. If you include the even more modest increases in 2001 and 2002, you still come up with a result that is nowhere close to the Lilith report’s original claim. The combined change from 1999 to 2002 is a rate increase of 26.9% - in other words, about half of what was originally reported.

So not only did the media take six years to correct the error in the Lilith report, they didn’t even get it right the second time around. But the story doesn’t end there...