2.B: What is a probability?

First, let's define probability. The probability that an event has a certain outcome is the fraction of times the event has that outcome. For example, a coin tossed in the air many times will land heads-up half the time and tails-up the other half of the time. Therefore, we say that the probability of heads is 1/2 and the probability of tails is 1/2.

Taking measurements in the lab can be described by a probability. Let's say that 100 members of a class were asked to measure the length of an object using a ruler. Because they would all hold and read the ruler slightly differently they would come out with slightly different answers. Hopefully, most of the answers would be grouped around a particular value. The probability distribution of the length measurements by our class for an object that is exactly 8cm in length might look like the plot that peaks near a value of 8cm.