(a) Examples of internal data are sales figures, results of previous research projects, product returns, customer complaints, etc. External data can be obtained from professional marketing research firms, such as competitors' market shares.

(b) Secondary data is data that is already available, such as previous studies. It can come from internal or external sources, such as government statistics on age and income distribution within a population or data from the Statista database. Primary data is collected for a specific question/decision problem that needs to be solved. For example, to determine which functions and features are preferred at which price levels.

(c) The advantage of secondary data is its low cost and immediate availability; the problem is that it is usually not a perfect fit for the problem at hand.