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# Re: st: spell variable according to a criteria

 From Abhimanyu Arora To statalist@hsphsun2.harvard.edu Subject Re: st: spell variable according to a criteria Date Wed, 4 Apr 2012 20:56:53 +0200

```Thanks very much indeed, Nick, was waiting for your ingenious take on
the problem.
Cheers
Abhimanyu

PS-Yes, I was wrangling with sorting for a couple of hours. I still
cannot comprehend fully but (different combinations of) sorting does
seem to influence the results. In fact I simplified the post as there
being just panel var and time, but actually the data is in the
mutilevel-form—country, city and time

On Wed, Apr 4, 2012 at 8:19 PM, Nick Cox <njcoxstata@gmail.com> wrote:
> I can't see a difference here in terms of what Stata will do between
> your first and second postings.
>
> In terms of your original problem, I see no need to -sort- each time
> around the loop as no command changes the sort order. In fact I see no
> need for a loop here at all.
>
> The problem appears to be this:
>
> Within panels, if -event- is 1 and -stretch- is # then -event- == 1 is
> the start of a spell with at most an extra # observations afterwards.
>
> Let's think about an -event- and suppose that -stretch- is 7. Then can
> we think of that of starting a count-down: 7, 6, 5, 4, 3, 2, 1, 0.
> When we get to 0, the spell is finished. (NASA used to say
> "Blast-off".)  But presumably a new -event- can reset the count-down.
>
> Here is sample code setting up a fake dataset.
>
> set obs 100
> gen panel_id = cond(_n < 51, 1, 2)
> bysort panel_id : gen time = _n
> gen event = 1 if  runiform() < 0.2
> gen stretch = ceil(18 * runiform()) if event == 1
>
> Now the algorithm is just this:
>
> bysort panel_id (time) : gen inspell = stretch if _n == 1
> by panel_id : replace inspell = max(stretch, inspell[_n-1]-1)  if _n > 1
>
> Now your spells are defined by -inspell- being >= 0 and not missing.
>
> Nick
>
> On Wed, Apr 4, 2012 at 5:06 PM, Abhimanyu Arora
> <abhimanyu.arora1987@gmail.com> wrote:
>
>> I think I managed.
>>
>> So in the line
>> bys panel_identifier:replace event=1 if  stretch[_n-(`i'+1)]>`i' &
>> stretch[_n-(`i'+1)]!=. & event==.
>>
>> I changed to
>>
>> bys panel_identifier:replace event=1 if  stretch[_n-`=`i'+1']>`i' &
>> stretch[_n-`=`i'+1']!=. & event==.
>>
>> in order to evaluate the local macro at every loop.
>
> On Wed, Apr 4, 2012 at 5:40 PM, Abhimanyu Arora
>
>>> I have a (monthly) time indicator and a variable 'stretch'  that
>>> identifies the spell of an event. I need to create a dummy in case the
>>> event took place or not.
>>> Currently event=1 for the start of the event (an missing for the
>>> rest). If stretch==1, event should equal 1 for the next time period.
>>> If stretch =2 event=1 for 2 more time periods (besides the starting
>>> time).
>>>
>>> I know that stretch ranges from 1 to 18 (but may not have the values
>>> in between).
>>>
>>> When I try without using the following loop (putting in the numbers
>>> manually that is), it seems OK, But with the following loop things
>>> aren't quite right.
>>>
>>> local i=0
>>> while `i'< 18 {
>>> bys panel_identifier:replace event=1 if  stretch[_n-(`i'+1)]>`i' &
>>> stretch[_n-(`i'+1)]!=. & event==.
>>> sort panel_id time
>>> local ++i
>>> }
>>>
>>> Where does the mistake lie?
>
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```