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# Re: st: Generate a more complex lagged variable

 From Nick Cox <[email protected]> To [email protected] Subject Re: st: Generate a more complex lagged variable Date Tue, 1 Feb 2011 16:56:13 +0000

```>3 should be >= 3

On Tue, Feb 1, 2011 at 4:48 PM, Nick Cox <[email protected]> wrote:
> You are referring to your code and not showing it to us, so quite what
> you did is unclear.
> Perhaps you tried something like
>
> gen H[2] = P[1]
>
> and as you say Stata doesn't like that.
>
> It seems that that you have panel data. Setting that aside you would have
>
> gen H = P[1] in 2
> replace H = a * H[_n-1]  + (1 - a) * P[_n-1] in 3/L
>
> In a panel context that would need to be something like
>
> bysort firmid (time) : gen H = P[1] if _n == 2
> by firmid : replace H = a * H[_n-1]  + (1 - a) * P[_n-1] if _n > 3
>
> A basic tutorial on -by:- and subscripts is accessible at
>
> SJ-2-1  pr0004  . . . . . . . . . . Speaking Stata:  How to move step by: step
>        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N. J. Cox
>        Q1/02   SJ 2(1):86--102                                  (no commands)
>        explains the use of the by varlist : construct to tackle
>        a variety of problems with group structure, ranging from
>        simple calculations for each of several groups to more
>        advanced manipulations that use the built-in _n and _N
>
> There would be some gain and no loss in using L.H and L.P after a
> Alternatively, in Mata subscripts behave more like what you
> (reasonably) seem to expect.
>
> Nick
>
> On Tue, Feb 1, 2011 at 4:30 PM, Ari Dothan <[email protected]> wrote:
>> Hi all,
>> In the formula H_t=a*H_t-1+(1-a)*P_t-1, P is an existing variable per firm/year.
>> Assuming that H[2]=P[1], and taking a=0.1: How can I build the column
>> of H_t by firm?
>> Since it understands [x] as weights, Stata indicates that "weights are
>> not allowed", so my problem becomes the specific calculation of each
>> observation in the H_t column.
>

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