This resource defines and explains the arithmetic mean using an example on employee salaries.
This resource gives 3 questions readers should ask when presented with data and why to ask them: Where did the data come from? Have the data been peer-reviewed? How were the data collected? This page also describes why readers should: be skeptical when dealing with comparisons, and be aware of numbers taken out of context.
This page calculates probabilities for a Poisson distribution.