A Rhythm Map is a heat map that displays time in the X and Y dimensions. They are an excellent way to visualize repeating patterns in time, such as how crimes occur by hour and data of week. Here we look at some interesting patterns in burglaries in the City of San Diego.
First, here is the map for a range of crime types in San Diego, compiled from the type, time and date of about 400K crime incidents in the City of San Diego from 2006 to 2012.
Each square is a crime type. The vertical axis is the hour of the day, and the horizontal axis is the day of the week, with Sunday being the cell between 0 and 1. Darker red means there are more crimes than lighter red and yellow. The colors are not comparable across squares, only within the cell. So, the dark red cell at 5:00PM on Friday in the Burglary square may represent a very different number of crime incidents than then dark red cell at 12:00AM on Thursday in Sex Crimes. Also note that these views combine citations, arrests and reported crimes, and there may be different patters when the maps are broken out on that factor.
There are a lot of interesting patterns here, but we’ll focus on Burglary. The first thing to notice is there are two time ranges, groups of darker red cells, when burglaries occur: during the work hours on weekdays and on Friday evenings. ( The strong line at noon is most likely an artifact of crimes for which the time is not known being given that value arbitrarily. )
What accounts for the two separate time ranges? First, let’s break it out by community. This chart uses Clarinova Place Codes for the community names.
Here we see that some communities exhibit one pattern or the other, and sometimes both. Downtown ( SanDOW ), La Jolla ( SanLAJ ) and Mira Mesa ( SanMIR ) show the Friday pattern, while Southeastern ( SanSOT), Greater North Park ( SanGRE) and Midtown ( SanMID ) show the week day pattern.
Community distinctions may explain some of the differences in the patterns, but there is a factor that is probably more important: residential vs commercial crime. So, let’s split out the maps on that factor.
Here is where the distinctions become the strongest. In Otay Mesa ( SanOAT ), Mira Mesa ( SanMIR ) University ( SanUNV ) and others, the Friday evening pattern completely splits from the weekday pattern. However, we also see a new weekday pattern in the commercial burglaries in Claremont ( SanCLA ), Uptown, Midtown, with commercial burglaries occurring across the weekday evenings.
Those features are consistent with exactly what you’d expect from burglary: the burglaries occur when the business and homes are unoccupied. But it doesn’t explain why in many communities the commercial crimes would occur more frequently on Friday evenings. Another unusual pattern is that in Pacific Beach ( SanPCF ) there is a residential burglary cluster on Friday and Saturday evenings, with a similar but weaker pattern occurring in Uptown and College.
Rhythms are a powerful way to look for patterns in time-structured data, because they take advantage of the ways that human brains most quickly process visual information. However, they aren’t a complete solution; they are just a start. Before making any recommendations based on the data, we’d want to do a few statistical tests, and at least, look at the absolute number of incidents per cell in the areas exhibiting patterns.