Duplicate when creating lag variable

[Phil_NZ]

Hi SAS community!

I face a problem when creating the lag variables with the duplicate value comes from nowhere, can you please give any suggestions about that? Many thanks

My have data is as below:

Type year obs amihud up_us nosh vo p_us
13016K	1987	0	.	.	.	.	.
13016K	1988	0	.	.	.	.	.
13016K	1989	0	.	.	.	.	.
13016K	1990	0	.	.	.	.	.
13016K	1991	0	.	.	.	.	.
13016K	1992	0	.	.	.	.	.
13016K	1993	0	.	.	.	.	.
13016K	1994	0	.	.	.	.	.
13016K	1995	0	.	.	.	.	.
13016K	1996	0	.	.	.	.	.
13016K	1997	0	.	.	.	.	.
13016K	1998	0	.	.	.	.	.
13016K	1999	0	.	.	.	.	.
13016K	2000	0	.	0.2945	40120	0.3	42.9638
13016K	2001	0	.	0.1945	40120	.	28.3783
13016K	2002	0	.	0.1802	45558	0.7	26.2883
13016K	2003	0	.	0.2712	45908	.	39.5715
13016K	2004	0	.	0.1568	45908	.	22.8729
13016K	2005	0	.	0.0337	45908	.	4.9228
13016K	2006	0	.	0.0394	45908	1.4	5.7496
13016K	2007	0	.	0.0483	97085	.	7.4443
13016K	2008	0	.	0.0349	100233	.	5.3736
13016K	2009	0	.	0.0351	120065	.	5.5743
13016K	2010	0	.	0.0205	152711	.	3.2582
13016K	2011	0	.	0.0195	308196	0.5	3.8958
13016K	2012	0	.	0.0093	308196	.	1.8687
13016K	2013	0	.	0.0018	308196	.	0.3579
13016K	2014	0	.	0.0368	315410	8.9	0.3683
13016K	2015	0	.	0.0255	1077004	.	0.2546
13016K	2016	4	0.0005137639	0.2498	159721	20.3	0.2498
13016K	2017	9	0.0000546129	0.1799	161321	198.4	0.1799
13016K	2018	2	0.0002143573	0.1619	161321	50	0.1619
13016K	2019	2	0	0.1687	177464	.	0.1687
130347	1987	0	.	.	.	.	.
130347	1988	0	.	.	.	.	.
130347	1989	0	.	.	.	.	.
130347	1990	0	.	.	.	.	.
130347	1991	0	.	.	.	.	.
130347	1992	0	.	.	.	.	.
130347	1993	0	.	.	.	.	.
130347	1994	47	0.0000738433	0.7	43242	22.5	0.64
130347	1995	35	0.0000662663	0.65	43242	.	0.59
130347	1996	25	0.0000750143	1.08	43242	566.1	0.99
130347	1997	44	0.0001311861	1.11	66105	.	1.11
130347	1998	15	0.0000954854	0.63	66806	2.5	0.63

and I create two lag variables of p_us and vo by the code below:

data want;
set have;
by Type;
if first.type then 
   do;
      lag_vo=.;
      lag_p_us=.;
   end;
else 
   do;
      lag_vo=lag(vo);
      lag_p_us=lag(p_us);
   end;
run;

And the result is as below:

Type	year	obs	amihud	up_us	nosh	vo	p_us	lag_vo	lag_p_us
13016K	1987	0	.	.	.	.	.	.	.
13016K	1988	0	.	.	.	.	.	.	.
13016K	1989	0	.	.	.	.	.	.	.
13016K	1990	0	.	.	.	.	.	.	.
13016K	1991	0	.	.	.	.	.	.	.
13016K	1992	0	.	.	.	.	.	.	.
13016K	1993	0	.	.	.	.	.	.	.
13016K	1994	0	.	.	.	.	.	.	.
13016K	1995	0	.	.	.	.	.	.	.
13016K	1996	0	.	.	.	.	.	.	.
13016K	1997	0	.	.	.	.	.	.	.
13016K	1998	0	.	.	.	.	.	.	.
13016K	1999	0	.	.	.	.	.	.	.
13016K	2000	0	.	0.2945	40120	0.3	42.9638	.	.
13016K	2001	0	.	0.1945	40120	.	28.3783	0.3	42.9638
13016K	2002	0	.	0.1802	45558	0.7	26.2883	.	28.3783
13016K	2003	0	.	0.2712	45908	.	39.5715	0.7	26.2883
13016K	2004	0	.	0.1568	45908	.	22.8729	.	39.5715
13016K	2005	0	.	0.0337	45908	.	4.9228	.	22.8729
13016K	2006	0	.	0.0394	45908	1.4	5.7496	.	4.9228
13016K	2007	0	.	0.0483	97085	.	7.4443	1.4	5.7496
13016K	2008	0	.	0.0349	100233	.	5.3736	.	7.4443
13016K	2009	0	.	0.0351	120065	.	5.5743	.	5.3736
13016K	2010	0	.	0.0205	152711	.	3.2582	.	5.5743
13016K	2011	0	.	0.0195	308196	0.5	3.8958	.	3.2582
13016K	2012	0	.	0.0093	308196	.	1.8687	0.5	3.8958
13016K	2013	0	.	0.0018	308196	.	0.3579	.	1.8687
13016K	2014	0	.	0.0368	315410	8.9	0.3683	.	0.3579
13016K	2015	0	.	0.0255	1077004	.	0.2546	8.9	0.3683
13016K	2016	4	0.0005137639	0.2498	159721	20.3	0.2498	.	0.2546
13016K	2017	9	0.0000546129	0.1799	161321	198.4	0.1799	20.3	0.2498
13016K	2018	2	0.0002143573	0.1619	161321	50	0.1619	198.4	0.1799
13016K	2019	2	0	0.1687	177464	.	0.1687	50	0.1619
130347	1987	0	.	.	.	.	.	.	.
130347	1988	0	.	.	.	.	.	.	0.1687
130347	1989	0	.	.	.	.	.	.	.
130347	1990	0	.	.	.	.	.	.	.
130347	1991	0	.	.	.	.	.	.	.
130347	1992	0	.	.	.	.	.	.	.
130347	1993	0	.	.	.	.	.	.	.
130347	1994	47	0.0000738433	0.7	43242	22.5	0.64	.	.
130347	1995	35	0.0000662663	0.65	43242	.	0.59	22.5	0.64

The problem here is there should be no value for lag_p_value for the Type 130347 in the year 1988 (because p_value in the year 1987 is "."), but from my result, lag_p_value of the Type 130347 equals to p_value of Type 13016K in the year 2019 (0.1687).

I am wondering if there is anything wrong with my code.

I also included my data below.

Many thanks.

[Kurt_Bremser · Accepted solution]

Never call a LAG() function conditionally, unless you know exactly what you are doing and want to perform special tricks with the lagged values.

The LAG() function adds a value to the FIFO chain only when it is called, so you never feed values at first.type, but keep the last value of the previous group.

Change your data step like this:

data want;
set have;
by Type;
lag_vo = lag(vo);
lag_p_us = lag(p_us);
if first.type
then do;
  lag_vo = .;
  lag_p_us = .;
end;
run;

Use of whitespace to make code more readable is recommended, as is proper indentation (only one step per functional level).