Measurement instruments are usually calibrated annually, people/companies do this because it is easy to schedule. This may not necessarily be the best approach to take, most test instruments will drift over time, the newer the instrument the more frequently it needs to be calibrated, until its ability to reproduce measurements over a period of time has been established and is well understood. A user might estimate a suitable recalibration frequency, based on previous experience of similar instruments used in similar environments, but it is only when the actual data from a series of successive calibrations of a particular instrument has been properly analysed that a meaningful recalibration frequency can be determined for that particular instrument.