Data Point: A data point is one particular number or item from a data set.

Data Set: A data set is simply a group of numbers. In formal mathematics, data sets are distinguished from each other by using brackets. A more formal mathematical definition allows a data set to contain other things besides numbers (such as letters, items, or even concepts and ideas). The following data set contains only the numbers 2, 5, and 7.

Distribution: A distribution is simply how the data points are clustered. Are they spread apart evenly, or do most of them cluster in the middle and fall off towards the edge like a bell-shaped curve? Two data sets may have the same mean or median, but having different distributions gives them radically different properties.

Mean: The mean (or arithmetic mean) is what most people are referring to when the say average. It is simply the total sum of all the numbers in a data set, divided by the number of different data points.

Median: The middle data point in a data set.

Mode: The most common data point in a data set. This is the value that occurs with greatest frequency.

Population: A population is all of the members contained within a group. In statistics, the population is the group you want your results to generalize about. For example, if you are studying a particular species of fish. such as a Yellow Fin Tuna, then your population is all Yellow Fin Tuna. Your population would not be all fish, nor would your population be all the different species of tuna.

Sample: A sample is all of the units or members that you have studied, drawn from a larger population. In our tuna example, researchers may have found 50 particular yellow fin tuna to study. The sample therefore would consist of 50 yellow fin tuna. As a researcher, you hope that your sample is as representative of your population as possible. The closer the sample represents the population, the stronger and more accurate an inference drawn from the sample will be. This is why you want a large sample to study from.

T-test: A t-test is a common statistical test used to compare two groups, typically two groups' means (the difference of two means divided by a measure of variability). A t-test takes into account the number of units in the sample.