# Some Statistical Definitions

When determining what is to be studied, it is important to know what the ** population** to be studied is.

Is it going to be all cats, all long haired cats, or all curly haired dogs?

To study the whole population you would use a ** census**.

However, most of the time, the population is too large, so a sample is chosen.

A ** sample** will be a subset of the whole population. There are several Sampling Techniques, which can also be combined.

: the sample is collected based on the location, e.g. all of one zip code.*Cluster*: the sample is convenient to the person conducting the study, e.g. people who walk into the grocery store.*Convenience*: each person, thing, etc. has an equal chance of being included in the sample, e.g. pulling a number from a hat.*Random*: the population is sorted into groups and then a percentage of each group is selected based on the population, e.g. 10% of the sample will be 40 year olds.*Stratified*: every nth item or person is selected, e.g. the 5th caller to the radio station.*Systematic*

When collecting the sample it is important to avoid ** sampling bias**. This is a bias that occurs when the sample does not represent the population. It may represent what the person conducting the study wants to find.