We may classify data types from three different perspectives: numerical versus categorical, time series versus cross sectional, and structured versus unstructured.

Numerical, or quantitative, data are values that can be counted or measured and may be discrete or continuous. Categorical, or qualitative, data are labels that can be used to classify a set of data into groups and may be nominal or ordinal.

A time series is a set of observations taken at a sequence of points in time. Cross-sectional data are a set of comparable observations taken at one point in time. Time series and cross-sectional data may be combined to form panel data.

Unstructured data refers to information that is presented in forms that are not regularly structured and may be generated by individuals, business processes, or sensors.