This paper is concerned with the influence of different estimation procedures on the data for real output by industry group of a number of O.E.C.D. countries. The authors have examined the methods and indicators used in preparing sector real output data and have tried to assess the effect of the different methods on the recorded changes of sector real output. The data for real output, employment and productivity are compared for the different sectors and countries. The comparison between sectors lays particular emphasis on the dichotomy between the services and non‐service sectors of the economy. In this comparison as well as in inter‐country comparison it is seen that the data are influenced to a considerable extent by different methods. The survey of estimation methods also shows the incidence of use of double deflation techniques and other methods in the different countries, and the extent to which quality change, output specification and valuation problems are reflected in the different meth