The Index of Production (IOP) is a National Statistics survey designed to measure changes in quarterly output for the Production Sector in NI. The sampling frame used is Inter-Departmental Business Register (IDBR). A stratified sampling technique known as the Neyman Allocation is used. The overall Index of Production is calculated by weighting together industry level indices that describe the activity of an industry sector using the Standard Industrial Classification (SIC). Weights are based on the relative share of regional GVA on an annual basis.