Historical data refers to data that has been collected over a period of time in the past. This type of data is often used in research to analyze trends, patterns, and changes over time. Historical data can come from various sources such as archives, government records, historical documents, and previous research studies. Researchers may analyze historical data to gain insights into past events, understand the causes of certain phenomena, or make predictions about the future based on past trends. This type of data can be valuable in a wide range of disciplines including history, sociology, economics, and environmental science.