In comparing the monthly per capita consumption expenditure variable (COPC) between the two rounds, why is there such a big change?

Three things are at work here:

1. The big difference is that IHDS-I is monthly per capita and IHDS-II is annual. The IHDS-II variable will be changed back to monthly during the next data update.

2. Price changes: There is a variable, DEFLATOR, in the public IHDS-II file; mean= .5453441 .

3. Economic growth.

For example, take the mean above for IHDS-I: 955.09 multiply by 12 months and divide by the average IHDS-II deflator, .5453441=21,016. The difference between 21,016 for IHDS-I and 27,155 for IHDS-II is a measure of economic growth.

You will need to do this separately for each household. Please note, there is not a hh “inflator” for the IHDS-I file. So, you can deflate the IHDS-II households by dividing by 12 months and multiplying by DEFLATOR. This will give IHDS-II totals in IHDS-I prices.