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Re: st: generating a seq() going backwards and forward from a given timepoint?


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: generating a seq() going backwards and forward from a given timepoint?
Date   Fri, 14 Dec 2012 12:00:04 +0000

Looking again at Ariel's question, he does want sequence numbers and
not dates shifted to a new origin, so my last part does not apply. In
that case, I would recommend a three step.

bys ID (date): egen period = seq() if date >= mdy(1,1,2012)
by ID : egen nmissing = total(missing(period))
by ID : replace period = -nmissing + _n - 1 if missing(period)

Nick

On Fri, Dec 14, 2012 at 9:59 AM, Nick Cox <njcoxstata@gmail.com> wrote:
> -seq()- as an -egen- function is part of official Stata but it has roots in
>
> STB-50  dm70  . . . . . . . . . . . . . . . . Extensions to generate, extended
>         (help egenodd if installed) . . . . . . . . . . . . . . . .  N. J. Cox
>         7/99    pp.9--17; STB Reprints Vol 9, pp.34--45
>         24 additional egen functions presented; includes various string,
>         data management, and statistical functions;
>         many of the egen functions added to Stata 7
>
> STB-37  dm44  . . . . . . . . . . . . . . . . . . . . .  Sequences of integers
>         (help seq if installed) . . . . . . . . . . . . . . . . . .  N. J. Cox
>         5/97    pp.2--4; STB Reprints Vol 7, pp.32--33
>         egen now has seq() function
>
> so its generality and flexibility are a test of my work. (Bizarrely a
> fragmentary reference to -seq- survives in the manual entry for
> -egen-, but that is scheduled for removal.)
>
> You can specify a negative argument to -from()-, but Ariel wants here
> is a starting value that may differ between panels. -from()- must be
> supplied with an integer argument, and although you can set it up that
> it sees the result of an expression that evaluates to an integer, that
> result must be a constant, which as in this problem is likely to be
> too inflexible for panels.
>
> So, there is a case for generalising -seq()- to accept more general
> expressions, or possibly for adding an -origin()- option. My own
> suggestion is that the latter would spoil the simplicity of -seq()-
>
> That said, I would just use
>
> gen period = date - mdy(1,1,2012) + 1
>
> here and I suspect that whenever people have wanted something like
> that it has seemed more natural to do it directly than to fire up
> -egen-.
>
> -seq()- is a fairly trivial function. It is arguable that its main
> feature is to support blocks of equal identifiers, which can be useful
> e.g. for ANOVA problems. My recollection is that its origins lie in
> wanting to use it and in reading about similar functionality in what
> was then just described as S.
>
> Nick
>
> On Fri, Dec 14, 2012 at 3:31 AM, Ariel Linden, DrPH
> <ariel.linden@gmail.com> wrote:
>
>> This is probably a very simple solution, but I am getting stuck. I'd like to
>> generate a running sequence going both backwards and forwards from a
>> specified timepoint. -seq()- is pretty straightforward (pun intended) for
>> going forward, but I can't seem to figure out the backwards part.
>>
>> Below I have and data with the "period" variable that I'd like to achieve
>> via code (I did this manually). For this example, I specified that
>> 01/01/2012 is the starting point and ran the following code:
>>
>> . display date("01/01/2012", "MDY")
>> 18993
>>
>> . bys ID: egen period = seq() if date>=18993
>> (48 missing values generated)
>>
>> How do I get the backward periods (-1 to -24)? There will be IDs with
>> varying numbers of periods available, so this must work via bysort ID,
>> anchored on the same starting date for everyone:
>>
>> ID      date    period
>> WC9     01/01/2010      -24
>> WC9     02/01/2010      -23
>> WC9     03/01/2010      -22
>> WC9     04/01/2010      -21
>> WC9     05/01/2010      -20
>> WC9     06/01/2010      -19
>> WC9     07/01/2010      -18
>> WC9     08/01/2010      -17
>> WC9     09/01/2010      -16
>> WC9     10/01/2010      -15
>> WC9     11/01/2010      -14
>> WC9     12/01/2010      -13
>> WC9     01/01/2011      -12
>> WC9     02/01/2011      -11
>> WC9     03/01/2011      -10
>> WC9     04/01/2011      -9
>> WC9     05/01/2011      -8
>> WC9     06/01/2011      -7
>> WC9     07/01/2011      -6
>> WC9     08/01/2011      -5
>> WC9     09/01/2011      -4
>> WC9     10/01/2011      -3
>> WC9     11/01/2011      -2
>> WC9     12/01/2011      -1
>> WC9     01/01/2012      1
>> WC9     02/01/2012      2
>> WC9     03/01/2012      3
>> WC9     04/01/2012      4
>> WC9     05/01/2012      5
>> WC9     06/01/2012      6
>> WC9     07/01/2012      7
>> WC9     08/01/2012      8
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