1.2 Variables

Identify observational units, variables, parameters, and statistics from a statistical study or data set.

• An observational unit is an item or individual from which a datum is collected.
• A variable is a characteristic that may change from one observational unit to another.
• Data collected on numerical and categorical variables measured on observational units, including photographs, sounds, videos, and text, can convey meaningful information.
• A parameter is a numerical attribute or summary of the variable of interest for a population.
• A statistic is a numerical attribute or summary of the variable of interest for a sample. The value of a statistic from a certain sample is often not equal to the unknown value of the population parameter but may provide the basis for making inferences about the population parameter.

Identify types of variables.

• A categorical variable, also called a qualitative variable, takes on values that are category names or group labels.
• A quantitative variable, also called a numerical variable, takes on numerical values for a measured or counted quantity and generally has units of measure.

Identify types of quantitative variables.

• A discrete quantitative variable can take on a countable number of values. The number of values may be finite or countably infinite, as with the whole numbers.
• A continuous quantitative variable can take on an infinite number of possible values within a given interval. The number of values the variable can take on is measurable but not countable. This variable can take on all possible values between any pair of values.