Stata interaction variable base year

Question

My data is in the following format:

clear all
input year  ID  var1    var2
2000    1   100 .
2001    1   200 0.2
2002    1   300 0.3
2000    2   500 .
2001    2   300 0.4
2002    2   400 0.1
end

I would like to create an interaction variable between var1 and var2 where var1 always takes the value of the first year where I have data (in this case year 2000 as the base year).

year	ID	var1	var2	var1xvar2
2000	1	100	.	.
2001	1	200	0.2	20
2002	1	300	0.3	30
2000	2	500	.	.
2001	2	300	0.4	200
2002	2	400	0.1	50

How can I achieve this?

Cybernike Cybernike · Accepted Answer · 2021-02-08T22:56:19

The following works for me:

bysort ID (year): gen wanted = var1[1] * var2

list, sepby(ID)

     +----------------------------------+
     | year   ID   var1   var2   wanted |
     |----------------------------------|
  1. | 2000    1    100      .        . |
  2. | 2001    1    200     .2       20 |
  3. | 2002    1    300     .3       30 |
     |----------------------------------|
  4. | 2000    2    500      .        . |
  5. | 2001    2    300     .4      200 |
  6. | 2002    2    400     .1       50 |
     +----------------------------------+

The [1] refers to the first observation in each ID panel, which in this case is the lowest value of year because the dataset is sorted by ID and year.

Stata interaction variable base year

1 Answers