Real-Life Math
You're examining a spate of armed robberies in a city with a large penitentiary. Based on your information, you wonder if former inmates with substance abuse problems pulled off the heists in order to buy drugs.
Profilers have to make educated guesses. That means understanding the law of averages as well as the law of the land. "Having basic statistical knowledge is very, very helpful when you're looking at data pertaining to a crime," says Brent Turvey. He is the author of a textbook on criminal profiling.
In the last century, criminologists placed offenders into broad categories called typologies. Through statistics, they developed a list of common characteristics for each typology.
Some profilers rely exclusively on this research to generate their profiles -- comparing unknown offenders to typologies to see what other characteristics they likely possess. Others use statistics to create a broad suspect pool before zeroing in on the likely culprit.
To see if your hunch is a solid one, you launch a study of people arrested for armed robbery in the last 5 years. You compile data on how many served a prison sentence and if their criminal records included convictions for narcotics offences.
The results come in. Out of 250 arrests for armed robbery, 180 served a prison sentence, while 162 had prior convictions for narcotics offences.
Now you need to take the raw data and present it as a percentage. Remember that your aim is to see how many people arrested for armed robbery in the past 5 years have been former inmates, and how many of those have prior convictions for narcotics offences.