Differences in the enterprises strata classification between sampling and calibration sources: the implications for Istat quarterly indicators on job vacancies and hours worked

The Italian National Institute of Statistics produces quarterly estimates of the job vacancy rate and hours worked for EU Regulations and national dissemination through calibrated weights. Job vacancy and hours worked data are collected by two direct business surveys. The survey samples are drawn from the Italian statistical business register (ASIA), updated to two years before the reference quarter of the survey data, while the calibration constraints are derived from an administrative based source for the reference quarter. The distance in time between the reference periods of sampling frame and source for calibration constraints implies that sample units may be classified as belonging to different strata depending on whether the information in the sample frame or in the source used for calibration constraints is used, due to changes in the enterprises’ economic activity and/or size between the two reference periods. As a consequence, if the initial weights are based on the sample frame units classification, calibrated weights are found to show a high variability within some calibration strata. In particular, in a given calibration stratum, weights can be concentrated only on few sample units while the remaining ones can be near to zero. To ensure a more homogeneous distribution of the calibration weights within strata, the initial weights have been recalculated, updating the sample units classification with the information used in the calibration strata construction and based on the source used for the calibration constraints. Furthermore, the initial weights are calculated so as to correct also for non-response by the inverse of the response rate in the calibration stratum. The first empirical results obtained applying this method have shown a reduction in the estimated sampling variance with respect to those obtained using initial weights based only on the not updated information of the sampling frame.