Does your organization report a weekly crime stat? Is part of that report a measure of how the week compares to the same week the year before? If so, it’s important that you understand how ISO week dates work so that your report offers an accurate comparison between this year and prior years. In this post I am going to first discuss what an ISO week date is and then I am going to explain how it helps create better crime stat reports.
First, what is an ISO week date? Hopefully you’re familiar with the International Organization for Standardization (ISO). They publish a lot of standards and ISO 8601 is the standard that deals with a calendar system that gives each week of the year a number. For example, this article was written on October 21, 2012, which, in ISO date week notation, is written 2012 W42 4. Breaking the date down: the first number is the year, the second number is the week of the year (42 in this case) and the third number is the day of the week (the 4th day is a Thursday because the system states that Monday is the first day of the week). Most of the time a year has 52 weeks, sometimes it has 53 weeks to handle leap years. According to Wikipedia the system is most often used in government and business for keeping fiscal years.
This is pretty straightforward to understand but the tricky parts comes when determining the first week of the year as the first week introduces slight discrepancies between the ISO system and the traditional Gregorian calendar that people are used too. To wit: ISO 8601 defines the first week as the week with the year’s first Thursday in it. Using 2012 as an example, the first Thursday of 2012 was January 5th and, recalling that the ISO week begins on Monday, this means that the first day of the ISO year, that is 2012 W01 1, was January 2nd, 2012. But what about January 1st, 2012? It was actually the last part of week 52 of 2011 or 2011 W52 7. Yes, I know, it’s weird, but I think its utility outweighs its weirdness.
If you’ve made it this far in the post you’re probably thinking: “This seems overly complicated, why should this concern someone reporting crime stats?” A valid question. Consider a weekly crime report that covers October 8th through October 14th from 0000 hours to midnight. That’s a whole week and conveniently it is also week 41 of 2012. Now let’s say that for comparison our theoretical crime report also tabulates the stats for October 8th through October 14th for 2011. Is there a problem with this? Does it matter that the 2011 numbers run from October 8th (a Saturday in 2011) to October 14th (a Friday in 2011) instead of Monday to Sunday like it does in 2012? Is it enough that we capture one of each weekday in a weekly crime report? Is it relevant that the Saturday from 2011 is part of ISO week 40 while the Saturday from 2012 is from week 41?
I think this last point is key. Calls for service have a definite seasonal trend which means that for any particular day of the week, Saturdays for example, the calls for service will vary according to the week of the year. This means that, historically speaking, the number of calls for service on the Saturday in week 41 are likely going to be consistently different than the number of calls for service on the Saturday in week 40 and the same goes for every other day of the week. Basically, days are not created equal and that means that if you want to compare apples to apples for crime reports you should really compare equivalent time periods. The easiest way to do this is, you guessed it, by using ISO week dates and comparing week 41 from 2012 to the week 41 from 2011. In our example, compare October 8th through October 15th 2012 to October 10th to October 16th 2011.
But is this reasonable? Why is the week the unit of analysis here? In non-leap years October 8th through 14th is always the 281st through 287th day of the year, isn’t there consistency in that that gets thrown out if we adopt the weekly approach? Perhaps but I don’t think day-of-year consistency is more useful than the weekly one. Most analysts recognize that calls for service have a distinct weekly pattern—there are more calls for service on Friday and Saturday then other days of the week—that is imposed upon the larger seasonal trend. By adopting the ISO week date system we can align reporting with this natural frequency found in the crime data and by locking into the frequency we can exploit it to make comparisons easier.
So what’s the bottom line? Because calls for service show a seasonal trend you should endeavor to compare the same weeks when performing a year over year analysis. And because calls for service show a weekly trend it makes sense to lock your reporting to a standard week that makes comparisons easy and straightforward. The best way to accomplish both of these goals is to adopt the established ISO week date system for weekly crime reports.