Pilot testing may prevent costly mistakes. Pilot testing is a trial run of procedures and instruments that you plan to use. For example, pilot testing of a mail survey to teachers could be done in a couple of different ways. You could mail the survey to a handful of teachers and then call them to discuss the questions. An easier, but less rigorous, method might be to distribute the survey after a professional development activity and ask the teachers there what they think.
The main purpose of pilot testing is to catch potential problems before they become costly mistakes. It is typically used if an instrument or method of data collection is being used for the first time or for the first time with a particular group (e.g., a survey used before with a different age group). Pilot testing provides information on how long data collection can be expected to take and a preview of how difficult items will be to complete. The latter is important as, with proper advanced notice, you can modify questions and possibly even the way you collect information (e.g., reading questions to people rather than having people read questions themselves).