How is industrial production measured?

Industrial production is measured in an index based on a reference period that expresses change in the volume of production output. Industrial production refers to the output of various industrial sectors like manufacturing, mining, electricity, air conditioning, steam, gas, etc. You can read about the Index of Industrial Production [UPSC Economy Notes] in the given link.

Further readings:

1. Merger of NSSO and Central Statistical Office (CSO) to form National Statistical Office (NSO)
2. Core Sectors of the Indian Economy