In international criminal law “Means, motive, and opportunity” is a popular summation of the three elements that need to be present for a crime to occur. Respectively they refer to:

a) Means: the ability of an individual to commit a crime,
b) Motive: the reason the individual had to commit the crime, and
c) Opportunity: whether or not the individual had the chance to commit the crime (Typically opportunity exists when a would-be criminal believes that the perceived reward is greater than the perceived risk.)

For example, if a criminal shot someone with a handgun and took his/her money when the victim was in an isolated, secluded area at night, the means would be the handgun, the motive financial (the money they stole), and the opportunity the fact that it would be unlikely someone else would witness or stop them.

Of these three elements “Opportunity” is the only one that can be controlled. This is achieved by creating the situation where the perceived risk of getting caught outweighs the perceived reward.

For example, if we consider the earlier scenario: If the victim avoided any isolated, secluded areas at night, choosing rather to stay in brightly lit, public areas, the criminal would not have the opportunity to commit the crime as it would be likely that somebody would witness them and stop them. So while the means and the motive are still there, no opportunity means – No Crime!

Our primary objective is therefore to prevent crime from happening by reducing criminal opportunities. This very simple philosophy can be applied to all areas of security risk management. 
 
SECURITY SYSTEM DESIGN

In designing a security system we look to the “4 D’s of Security”: Deter, Detect, Delay and Detain.

•  Deter:
With “Opportunity Reduction” in mind, the first element of an effective security solution is to deter the criminal. This is achieved by creating the situation where the perceived risk outweighs the perceived reward. Click here to read "In the mind of a house robber".

•  Detect:
Early detection of a threat can make a vital difference, sometimes a life or death difference. A potential intruder is more likely to flee empty handed if the alarm is raised before he gains entry, rather than once inside the property. One of the fastest growing criminal moudus operandi is “gate crashing”, the success of which is based on the speed of the criminals.
(more about gate crashing)

• Delay:
If you haven’t deterred him, and you haven’t detected him, you need to delay him! This is achieved with good quality barrier security (gates and burglar bars). The objective is to buy time, the more difficult it is for thieves to force through in order to gain access to your home, the less likely they are to try.

• Detain:
The final piece of the security puzzle is armed response. If, in the very likely event an intruder does manage to gain access to your property you’re going to need help - this is the role of an armed response service.