Whether you're talking about the theft of a laptop or someone hijacking your Mac to serve as a chat server, the vast majority of security threats any computer user faces are random. Someone can happen to be looking for a guy walking down this block with a computer carrying case or probing every IP address on a network looking for a vulnerability. Generally speaking, an attacker goes through less effort to obtain a computer or its data if it's not known to contain something of particular value (per the previous point). Even so, it's clear that your risk of randomly having your laptop stolen is higher if you're walking down a crowded city street than if you're walking down a quiet country road, if only because the number of people you come in contact with is higher.
Judging one's level of risk is a bit of a guessing game; no matter how carefully you assess statistical risks, there's always a chance that you, your computer, or the packets you send across the Internet may be in the wrong place at the wrong time. Nevertheless, some general principles apply that can serve as a rough guide while you consider the security procedures in this book. All things being equal, it makes sense to put more effort into security if your situation puts you at a higher risk.
Keep the following factors in mind:
Proximity to potential attackers. In terms of physical security — particularly the threat of theft or physical vandalism — your environment is of course the main ...