TECHNICAL NOTE The Employment Cost Index (ECI) measures the change in the cost of labor, free from the influence of employment shifts among occupations and industries. For information on survey concepts, coverage, methods, nonresponse adjustment, and imputation see National Compensation Measures Handbook of Methods at www.bls.gov/opub/hom/ncs/home.htm. Sample size Data for the March 2018 reference period were collected from a probability sample of approximately 27,300 occupational observations selected from a sample of about 6,600 private industry establishments and approximately 8,000 occupational observations selected from a sample of about 1,400 state and local government establishments that provided data at the initial interview. Standard errors To assist users in ascertaining the reliability of Employment Cost Index (ECI) series, standard errors of all current quarter estimates except seasonally adjusted series are also available, see www.bls.gov/ncs/ect/ectvar.htm. Standard errors provide users a measure of the precision ofan estimate to ensure that it is within an acceptable range for their intended purpose. Historical listings Historical listings are available at www.bls.gov/ect/#tables. The continuous occupational and industry series listing uses the Standard Industrial Classification Manual and Occupational Classification System series from 1975 through 2005 and the North American Industry Classification System and Standard Occupational Classification from 2006 to the present. It provides the official series deemed continuous after the change in classification systems. For more information on the criteria used in defining continuous series, see the article published in the Monthly Labor Review at www.bls.gov/opub/mlr/2006/04/art2full.pdf. Employer Costs for Employee Compensation data The costs per hour worked of compensation components are published in a separate news release titled "Employer Costs for Employee Compensation" (ECEC). The ECEC release dates are available at www.bls.gov/schedule/2018/home.htm. Historical ECEC data are available in summary documents at www.bls.gov/ect/#tables. Since the ECEC is calculated with current employment weights rather than the fixed weights used in computing the ECI, year-to-year changes in the cost levels usually differ from those in the ECI. Fixed Employment Weights For additional information on the use of fixed employment weights in computing the ECI, see www.bls.gov/opub/mlr/2016/article/introducing-2012-fixed-employment-weights-for-the-employment-cost-index.htm.