The Index of Services (IOS) is a National Statistics survey designed to measure changes in quarterly output for the services sector. The sampling frame used is Inter-departmental Business Register (IDBR). A stratified sampling technique known as the Neyman Allocation is used. Each stratum is indexed currently using 2013 = 100 as the base year. The overall Index of Services 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.