Murphy's Law can be stated as If something can go wrong, it will go wrong.

An analysis of common Murphy's Law cases shows how any model needs to be evaluated carefully before making decisions about the model (causation, etc.).

Here are some corollaries of Murphy's Law.

• Anything that can go wrong, will go wrong.
• You will always find something in the last place you look.
• No matter how long or how hard you shop for an item, after you've bought it, it will be on sale somewhere cheaper.
• The other line always moves faster.
• ... and so on ...

Murphy's Law was originated at Edwards Air Force Base in 1949, being named for Capt. Edward A. Murphy, an engineer working on Air Force Project MX981, a project that was designed to see how much sudden deceleration a person can stand in a crash.

For more information, see http://www.murphys-laws.com/murphy/murphy-true.html (as of 1972).

Is there a quantitative basis for Murphy's Law?

• Matthews, R. (April, 1997). Why things go wrong, Scientific American, p. 88-91.
• Murphy III, E. (August 1997). Letter to the editor. Scientific American, p. 8.

A queue is a waiting line.

Murphy's Law of queues:

• The line next to you will usually finish first.

Why does it always seem that every time you go grocery shopping, you are not in the fastest line?

Suppose that you enter the supermarket, fill your shopping cart, and enter one of five possible checkout lines.

Assume that there are no ties (e.g., if time is measured, ties are unlikely), and that you have no other knowledge about the problem.

Answer: What is the probability that the line you get in is not the fastest line? Answer: You have a 1/5 chance of being in the fastest line. Therefore, you have a 4/5 chance of not being in the fastest line. Question: Suppose that you choose one of the middle lines. What is the probability that, considering your line and the lines to either side, the line you pick is not the fastest line? Answer: You have a 1/3 change of being in the fastest line. Therefore, you have a 2/3 chance of not being in the fastest line. Question: Why does it always seem that every time you go grocery shopping, you are not in the fastest line? Answer: It is not just bad luck that you are not in the fastest line because, from a probability point of view, the odds are against you being in the fastest line.

Murphy's Law of maps:

• If a place you're looking for can lie on the inconvenient parts of the map, it will.

An inconvenient part of the map is a part near the edge.

The green area represents 50.0% of the total area of the map.

The red area represents 50.0% of the total area of the map.

Half of the time, the place you're looking for will lie on an inconvenient part of the map.

Murphy's Law of odd socks:

• If odd socks can be created, they will be.

Murphy's Law of umbrellas:

• Carrying an umbrella when rain is forecast makes rain less likely to happen.