Differenzvariable über mehrere Zeilen

[Hannah Zllnr]

Hallo zusammen,

ich versuche gerade eine Differenzvariable aus zwei Variablen zu erstellen.

Die Differenz soll aus zwei Variablen je einer ID gebildet werden und das für alle IDs -
also quasi ID1 Wert Variable Z1 - ID1 Wert Variable aus Zeile 2,
ID2 Wert Variable Z1 - ID2 Wert Variable aus Zeile 2 ...

id	wave	var1	var2
1001	2	1,2	.
1001	4	.	1,4
1002	2	2,0	.
1002	4	.	2,8

Da es zwei Wellen (Welle2 und Welle4) gibt, hat die Variable immer auf einer Welle ein Missing, da sie zu zwei verschiedenen Zeitpunkten abgefragt wurde.
Diese Befehle:
gen diffwarmth_pacs = warmth_pacsw2 - warmth_pacsw4

bysort id: gen diffwarmth_pacs = warmth_pacsw4 - warmth_pacsw2

Nun meine Frage, welcher Befehl kann eine Differenzvariable erstellen, in der die Werte var1 von den Werten var2
abgezogen werden?

Vielen Dank schonmal!

[Marvin Buermann]

Dear Hannah,

just as a small note: in general, you are more likely to get an answer in this forum if you ask in English.

Turning to your question, one solution in this particular case is:

xtset id wave
gen diff = var2 - l2.var1

It calculates the difference for var2(t=0)-var1(t=-2). You could also generate a new variable which maps the value for var1 to t=0 by "gen var1_wave4 = l2.var1" (only possible if previously "xtset") and then calculate the "normal" difference via "generate".

Best,
Marvin

[Hannah Zllnr]

Dear Marvin,

thank you so much for your answer and the code, it worked!

May i ask you a further question?
I would like to know how to subtract values within one line from another.
Moreover, this command should be done for the whole line (approx. 500 obeservations) of var 1. Do you have any idea for a code that could perform the subtraction of two values of one id for the whole line?

id	var1
1001	1
1001	0,8
1002	2
1002	1,2

Thanks and best wishes,
Hannah

[Nick Cox]

bysort id : gen wanted = var1[2] - var1[1]

is one answer to #3, but much depends on whether there is some other variable which defines a unique order within id.

[Hannah Zllnr]

Thank you Nick, the code worked perfect.