The purpose of perform stability study is that establish, based on testing a minimum of three batches of the drug substance and evaluating the stability information (including, as appropriate, results of the physical, chemical, biological, and microbiological tests), a re-test period applicable to all future batches.
here mainly seen that temperature, humidity, light effect.
all that study find out how log time drug product are stable
stability main also main part of Drug Development and Industrial Pharmacy