Formula Does Matter

Amid the chaos around the new gross domestic product figures put out by the Central Statistics Office, profound improvements made in another prominent statistical marker--the Consumer Price Index--also compiled by the CSO, went unnoticed. The CPI series was revised to a more recent base year, 2012. This alone probably deserves commendation as base years of other national price indices have grown significantly remote. It is indeed a tectonic shift in estimation procedure and conforms to international best practice concerning CPI. Here we discuss the decision of CSO to replace the arithmetic mean with the geometric mean in the updated CPI indices by going through each of the established approaches to index number theory used to identify the appropriate formulation to calculate basic indices.