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Almost everyone I know locks their car doors when they leave their vehicle for an extended time. However, I come from a country town and most people in that country town don't lock their car doors and rarely lock up their houses.

I moved to the city to live when I was 8 years old and have always wondered what the point is in locking your car doors? If someone can start your ignition without a set of keys they can surely, with even less effort, open your car doors.

Is there any evidence to suggest that locking your car door reduces the chance of your car being stolen or reduces the chance of a valuable being stolen from an out-of-sight area (glove box, car boot)?

Examples of the claim that people should lock their doors to prevent vehicle theft:

Tips to help you prevent vehicle theft: Close all windows and lock all doors before leaving your car unattended.

Lock your car. Approximately 50 percent of all vehicles stolen were left unlocked.

How To Help Prevent Vehicle Theft: Lock your vehicle every time you leave it, even in the driveway or garage of your home.

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The big threat that locking protects against is casual theft. (After all, who wouldn't want my ten year old road atlas and the change in my ashtray?) But even a thief who can jimmy open my car door takes a bigger risk of being seen doing it. He may look for easier targets... –  dmckee Aug 7 '12 at 0:09
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The doors protect the contents of your car, whereas the ignition protects the car itself. Can you provide an example of the claim that door locks protect the car, in stead of simply its contents? –  Sklivvz Aug 7 '12 at 6:51
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@warren Statistically you are wrong there, unless you can show that 50% of cars area left unlocked. If 90% of cars are locked and 50% of stolen cars area unlocked then leaving your car unlocked seriously increases the chance of theft. –  DJClayworth Aug 7 '12 at 18:28
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@warren I think you may want to reread your statistics textbook. –  DJClayworth Aug 7 '12 at 19:36
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@war Assume a parking lot with 10 unlocked & 990 locked cars. 20 cars are stolen, 10 locked and 10 unlocked, then 50% is very significant, as it's a 100% chance of having an unlocked car stolen versus a ~1% chance of having a locked car stolen. If instead we had a population of 400 unlocked & 600 locked cars, with 20 stolen cars, we'd have 2.5% unlocked & 1.7% locked cars stolen. Not very significant. But if we had 800 stolen cars out of the same parking with the same ratio, the chances would change to 100% vs 67%. So, one needs all the numbers to make sense of such a claim! –  Sklivvz Aug 7 '12 at 20:05

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