Converting Time Series Data to Different Periodicities Statistical agencies may report time series data on a monthly basis. This data can be converted to quarterly data or annual data by using a suitable conversion method. For example, a monthly price index can be converted to a quarterly price index by computing 3-month averages. Monthly data on the volume of retail sales can be converted to quarterly or annual data by summation. With SHAZAM, the SUM function on the GENR command can be used to convert time series data to different periodicities. The command format is: GENR newvar=SUM(var,n) The SUM function sums up n successive observations on the variable var starting at observation 1. Example Suppose monthly data for variable VARM is available for the sample period 1986M1 to 1996M12 (132 monthly observations). The SHAZAM commands below convert the data to quarterly data by using 3-month averages. SAMPLE 1 132 READ (file) DATEM VARM * Set the sample to the number of quarterly observations SAMPLE 1 44 * Convert to quarterly data by computing monthly averages GENR VARQ=SUM(VARM,3)/3 * Generate a quarterly date variable starting in 1986Q1 TIME 1986 4 DATEQ * Write the results to a data file WRITE (newfile) DATEQ VARQ STOP The WRITE command is used to save the new data to a data file. [SHAZAM Guide home]