Creating a time-varying variable with Panel data

[BJ Cunningham]

Hi - I am hoping someone will be able to help me with panel data! I am trying to create a time-varying variable (i.e., time between assessment points).
I am able to generate 0 for assessment #1 using: replace Time between assessment = 0 if Assessment#==1, but am not sure how to get Stata to calculate the difference in ages (the cells in bold & italics).

ID	Score	Assessment #	Age (months)	Time between assessment
1	20	1	18	0
2	30	1	20	0
2	35	2	26	*Should be 6 (26-20 mos.)
3	15	1	24	0
4	7	1	30	0
5	10	1	26	0
5	16	2	30	*Should be 4 (30-26 mos.)
5	18	3	37	*Should be 7 (37-30 mos.)
6	40	1	21	0

Thanks in advance for your suggestions!

[Attaullah Shah]

You can try

bys ID: gen dif = Age - Age[_n-1] if Assessment !=Assessment[_n-1]

I do not think the # string in the assessment variable is possible

[Nick Cox]

Thanks for the data table. It is usable but an example created with dataex (SSC) would need less engineering.

See http://www.statalist.org/forums/help#stata for more guidance.

clear
input ID    Score    Assessment Age Wanted 
1    20    1    18    0    
2    30    1    20    0    
2    35    2    26    6 
3    15    1    24    0    
4    7    1    30    0    
5    10    1    26    0    
5    16    2    30    4 
5    18    3    37    7 
6    40    1    21    0
end 

bysort ID (Age) : gen Suggested = cond(Assess != Assess[_n-1] & _n > 1, Age - Age[_n-1], 0) 

assert Wanted == Suggested 

list, sepby(ID) 

     +-------------------------------------------------+
     | ID   Score   Assess~t   Age   Wanted   Sugges~d |
     |-------------------------------------------------|
  1. |  1      20          1    18        0          0 |
     |-------------------------------------------------|
  2. |  2      30          1    20        0          0 |
  3. |  2      35          2    26        6          6 |
     |-------------------------------------------------|
  4. |  3      15          1    24        0          0 |
     |-------------------------------------------------|
  5. |  4       7          1    30        0          0 |
     |-------------------------------------------------|
  6. |  5      10          1    26        0          0 |
  7. |  5      16          2    30        4          4 |
  8. |  5      18          3    37        7          7 |
     |-------------------------------------------------|
  9. |  6      40          1    21        0          0 |
     +-------------------------------------------------+

[BJ Cunningham]

Thank you both so much. I appreciate the guidance!