The behaviour of the US consumer price index 1913–2003: a study of seasonality in the monthly US CPI

Abstract

This article analyses the seasonality in the monthly consumer price index (CPI) over the period January 1913 to December 2003. We examine three types of month effects: if the mean of monthly CPI changes of the entire data set, and of a given month were significantly different from zero; if the mean of monthly CPI changes of a given month was different from the mean of the other months; and if the variance of the monthly CPI changes for a given month was different from the variance of the other months. The mean of monthly CPI changes for the entire data set (0.27%) was found to be significantly greater than zero. The means of monthly changes show a downward trend from September to December. When the data are sliced into three sub-periods, we find an increasing trend in the means and medians of monthly changes but a decreasing trend in the SDs of the monthly changes. The mean of monthly CPI changes during the republican presidencies (0.15%) was significantly lower than during the democratic presidencies (0.38%).