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Wednesday, 30 December 2009

More Poisson

Following on from my last post I have carried an analysis of the grid square that had the most street robbery allegations recorded on the MPS crime recording system - CRIS. In the financial year 2007-8 45 allegations were recorded giving an average of 0.9 allegations per week. I allocated each allegation to the week in which they occurred.

20 weeks had no allegations
21 weeks had one allegation
11 weeks had 2 or more allegations

This results in the percentages of;

0 = 38.5%
1 = 40.4%
2 or more = 21.2%

The expected values for a Poisson distribution are;

0 = 40.7%
1 = 36.6%
2 or more = 22.8%

Therefore the answer to the question did the occurrence of street robberies from week to week in a specified 250 metres by 250 metres grid square in a London Borough in financial year 2007-8 follow a Poisson Distribution is "Yes".

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