# Change is the only constant.

A variable is any characteristics, number, or quantity that can be measured or counted. A variable may also be called a data item. Age, sex, business income and expenses, country of birth, capital expenditure, class grades, eye colour and vehicle type are examples of variables.

# Variables

PentaCalc Pro can calculate many one variable statistics. Many of these statistics such as Sum of x and Sum of x squared are useful for other statistical calculations. Other one variable statistics are useful by themselves, such as the mean and standard deviation.

# Standard Deviation

There are two types of standard deviation PentaCalc Pro is capable of calculating. [sx] or [sy] calculates the sample standard deviation and [œx] or [œy] calculates the population standard deviation. The sample standard deviation assumes that the data you have entered are a sample of a large population for which you are trying to estimate the standard deviation.

# Population standard deviation assumes that the data you have entered are the entire population

In the case of the car mileages (from the following example), the population standard deviation is the one you would want to calculate, because you own only 10 cars and those 10 cars are all you are interested in (they are the entire population). If your calculations were for the benefit of the EPA (to estimate the mileage for a certain model car based on a sample of 10 road tests) the sample standard deviation would be your deviation of choice, because the EPA would take your results and apply them to all cars of that model.