Probability represents how likely an event is to occur. The probability of an outcome can be measured as \[ P = \frac{\#\ Outcomes}{\#\ Trials}\]

To get a real measure of this, we have to observe an infinite number of ** trials**, which is impossible.

Instead, we are stuck merely observing a ** sample** of possible trials.

We try to make this sample as representative as we can, but sometimes logistics and chance conspire to make a sample of trials unrepresentative.

If the sample of trials is unrepresentative, then our estimate of probability may be badly wrong.