Random variable
A random variable is a variable that takes an uncertain value.
Since the event that takes place generating a particular value to the
random variable is uncertain, some probability of occurrece can be
attached to it.
A random variable is discrete if it can only assume some numerical
values. Example: the results in a test with 10 questions can be
0,1,2,...,10.
A random variable is continuous when it can assume any value in an
interval. Example: time between student arrivals at a professor's office
during office hours (in minutes).