Re: st: how do you subtract the value of one group from the value of another group?

[Ken Ning <kenscholar2008@gmail.com>]

Hi Nick,

Thanks a lot for getting back to me.

In fact, i have an unbalanced panel data and I calculate a value for
each year in each group. I want the same group (e.g. group 1) in year
2 (e.g 2002) to have the value of the year 1 (2001) subtracted from it
within the same group (i.e. 9-7, and 6-9) and repeat the same process
for group 2 (12-11, 11-10 etc)



group    year    value
1    2001    7
1    2001    7
1    2001    7
1    2001    7
1    2002    9
1    2002    9
1    2002    9
1    2002    9
1    2003    6
1    2003    6
1    2003    6
1    2003    6

2    2001    10
2    2001    10
2    2001    10
2    2002    11
2    2002    11
2    2002    11
2    2003    12
2    2003    12
2    2003    12

Many thanks

Ken




On 19 August 2013 21:36, Nick Cox <njcoxstata@gmail.com> wrote:
> That wasn't my suggestion, which was geared to your example with just 2 groups.
>
> Now you tell us you have many groups. You need to tell us how
> differences are defined.
>
> Your code looks _legal_, so I don't understand the error message.
>
> If people don't state the real problem, poor solutions are all too likely.
>
> Nick
> njcoxstata@gmail.com
>
>
> On 19 August 2013 21:05, Ken Ning <kenscholar2008@gmail.com> wrote:
>> Hi Nick,
>>
>> Thanks a lot for this.
>>
>> As I have many groups， I tried this following your suggestion
>>
>> bysort year (id) : gen diff = value[_n] - value[_n-1]
>>
>>
>> but it says "factor variables and time-series operators not allowed"
>>
>> What did i do wrong?
>>
>> Many thanks
>> Lutao
>>
>>
>>
>> On 19 August 2013 20:43, Ken Ning <kenscholar2008@gmail.com> wrote:
>>> Hi statausers
>>>
>>> how do you subtract the value of one group from the value of another group?
>>>
>>> e.g. if i want to substract the value of group 2 (value= 11 in 2001
>>> and 12 in 2002) from group1 (value= 7 in 2001 and 8 in 2002) year by
>>> year? so this needs to be 11-7 and 12-8 and listed next to the value
>>> of group 2
>>>
>>>
>>>
>>> id year value  (minus value)
>>> 1 2001   7     .
>>> 1 2001   7    .
>>> 1 2001   7   .
>>> 1 2002    8  .
>>> 1 2002    8  .
>>> 1 2002    8  .
>>>
>>> 2 2001   11     x (need to get)
>>> 2 2001   11     x
>>> 2 2001   11     x
>>> 2 2002   12     x
>>> 2 2002   12     x
>>> 2 2002   12     x
>>>
>>> Many thanks
>>> Ken
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