(edited) I am estimating a Central Bank reaction function using GMM using monthly data.
Following the literature, I used lagged values of each variable as instruments (are there other feasible options?)
Variables: gdp growth monthly forecast, monthly cpi inflation, difference between domestic interbank market and federal funds rate, exchange rate, oil price. All data are on monthly basis, and for frequent data I use the value at the end of the month. Deviation are computed with a one-sided HP filter, a part inflation, where I just subtract the inflation target from cpi inflation.
How could I chose the optimal lag? I thought to fit each variable in a AR model, use information criteria to chose the lag, and then use this lag number for the instruments.
For data in which I have more frequent data, could I use daily lags as instruments? Let us say that in my equation I have the exchange rate, of which I have daily data. The variable I want to instrument is "exchange rate the last day of the month" Could I use as instrument "exchange rate one day before the end of the month", "exchange rate two days before the end of the month" and "exchange rate three days before the end of the month"?
Up to now as instrument for the "exchange rate the last day of the month" I used "exchange rate the last day of the month before", "exchange rate the last day of two months before", and exchange rate the last day of three months before"
Thanks for your help!