Lagging variables in unbalanced panel

[GKati]

Hello, 

I have an UNBALANCED PANEL  of clients (i.e. not all clients are present on every year) for four years (2010-2013). I would like to calculate the change in costs over the years. The problem is that when I lag the variable "costs", SAS lags over all client numbers (i.e. 2013 of client1 will become 2010 for client2). How do I prevent this from happening?

I can't just take out year=2010, because for some clients 2011 was the first year.

This is what I have so far:

data costchange;

set sample;

by cient;

lcost=lag(cost);

cost_score = (cost - lcost)/lcost;

where treated=1;

run;

[RW9]

As always, some test data in the form of a datastep would help clarify what you mean.  At a guess, maybe use retain'd variables and set them once per group, then when missing use the retained variable.

[Astounding · Accepted solution]

When using LAG, it's important to make sure it executes on every observation.  You're doing that ... so far so good.

The trick is to wipe out the value for observations that begin a new CLIENT.  So:

data costchange;

set sample;

by cient;

lcost=lag(cost);

if first.client then lcost = . ;

cost_score = (cost - lcost)/lcost;

where treated=1;

run;

[GKati]

Perfect! Just what I was looking for.