# Variables

While a “variable” in algebra really just means one thing–an unknown value–you’ll come across dozens of types of variables in statistics. Some are used more than others. For example, you’ll be much more likely to come across continuous variables than you would dummy variables. Click on a variable name to learn more about that particular type.

## Common Types of Variables

• Categorical variable: variables than can be put into categories. For example, the category “Toothpaste Brands” might contain the variables Colgate and Aquafresh.
• Confounding variable: extra variables that have a hidden effect on your experimental results.
• Continuous variable: a variable with infinite number of values, like “time” or “weight”.
• Control variable: a factor in an experiment which must be held constant. For example, in an experiment to determine whether light makes plants grow faster, you would have to control for soil quality and water.
• Dependent variable: the outcome of an experiment. As you change the independent variable, you watch what happens to the dependent variable.
• Discrete variable: a variable that can only take on a certain number of values. For example, “number of cars in a parking lot” is discrete because a car park can only hold so many cars.
• Independent variable: a variable that is not affected by anything that you, the researcher, does. Usually plotted on the x-axis.
• A measurement variable has a number associated with it. It’s an “amount” of something, or a”number” of something.
• Nominal variable: another name for categorical variable.
• Ordinal variable: similar to a categorical variable, but there is a clear order. For example, income levels of low, middle, and high could be considered ordinal.
• Qualitative variable: a broad category for any variable that can’t be counted (i.e. has no numerical value). Nominal and ordinal variables fall under this umbrella term.
• Quantitative variable: A broad category that includes any variable that can be counted, or has a numerical value associated with it. Examples of variables that fall into this category include discrete variables and ratio variables.
• Ratio variables: similar to interval variables, but has a meaningful zero.

## Does all data have a median, mode and mean?

Yes and no. All continuous data has a median, mode and mean. However, strictly speaking, ordinal data has a median and mode only, and nominal data has only a mode. However, a consensus has not been reached among statisticians about whether the mean can be used with ordinal data, and you can often see a mean reported for Likert data in research.