In sociology, secondary data refers to data that is already available to the sociologist as opposed to primary data which they produce themselves. This might include official statistics, for example. An advantage of secondary data is its practicality: conducting primary research is time-consuming and difficult and can be expensive, so if relevant data already exists, why not use it? However, the researcher has no control over how that data was produced. It will have been produced for a purpose and it is very unlikely to have been for the purpose it is now being put. For example, statistics might be produced for political reasons and be subject to manipulation or bias.