Primary sources
A primary source of information is one that provides data from an original source document. This may be as simple as an invoice sent to a business or a cheque received. It may be more complex, such as a set of sales figures for a range of goods for a tinned food manufacturer for one week, or it may be a set of sales figures over several weeks and several locations. There are many examples of primary sources in many walks of life, but generally a primary source is defined as being where a piece of information appears for the first time.
Secondary sources
A secondary source of information is one that provides information from a source other than the original. Secondary sources are processed primary sources, second-hand versions. Examples of secondary sources could be an accounts book detailing invoices received, a bank statement that shows details of cheques paid in and out. Where statistical information is gathered, such as in surveys or polls, the survey data or polling data is the primary source and the conclusions reached from the survey or the results of the poll are secondary sources.
Tertiary sources
Tertiary sources consist of information which is a distillation and collection of primary and secondary sources.