Multiple lags in Panel data

[namrata]

Hi

I wanted to create lags for a few variables in a panel dataset.

It seems the straightforward approach(below) works for me.

data funda1;

set funda;

by gvkey year;

diff = dif (Year);

mve=shares*price

mve_lag=lag(mve);

if first.gvkey then mve_lag=.;

if diff > 1 then mve_lag=.;

tobinq = (mve+at-ceq-txdb)/at;

tobinq_lag=lag(tobinq);

if first.gvkey then tobinq_lag=.;

if diff > 1 then tobinq_lag=.;

at_lag=lag(at);

if first.gvkey then at_lag=.;

if diff > 1 then at_lag=.;

run;

data funda2;

set funda1;

by gvkey year;

if at_lag='.' then delete;

if tobinq_lag='.' then delete;

if mve_lag='.' then delete;

if funded_lag='.' then delete;

capx_s=capx/at_lag;

rd_s=rd/at_lag;

if capx_s ='.' then delete;

if rd_s='.' then delete;

if year lt 1990 then delete;

if year gt 1998 then delete;

if dif(year) ne 1 or gvkey ne lag(gvkey) then group+1;/* this step and proc sql below is to ensure atleast 3 consecutive year observations for all firms in dataset*/

run;

Now in the above code,I have specified the command for deleting missing first lags(for instance:"if at_lag='.' then delete") in the second data step. When I put that statement immediately after every step of lag creation(i.e., after" if diff > 1 then mve_lag=.;"), the first lag of each variable seems to take on the lag_value of the previous gvkey and does not return as '.'.

This might be a very simple question but I am curious as to why does that happen. Could anybody please explain?

Thanks.