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RE: st: FW: Max/Min and Growth rates among subsample(by year) of panel data


From   Nickolas Lyell <[email protected]>
To   "[email protected]" <[email protected]>
Subject   RE: st: FW: Max/Min and Growth rates among subsample(by year) of panel data
Date   Fri, 16 Aug 2013 14:44:38 -0400

Perfect, thank you.


Nicholas Lyell
Research Associate
National Association of Counties | NACo
[email protected] | 202.661.8820


-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Nick Cox
Sent: Friday, August 16, 2013 2:43 PM
To: [email protected]
Subject: Re: st: FW: Max/Min and Growth rates among subsample(by year) of panel data

That's possible with the same trick.

bysort fips : egen RGDP1990  = max(RGDP / (year == 1990))

Here the use of -max()- may seems strange, but there is only one value for each county, and so -max()- just returns that one value: you could use -max()-, -min()-, -mean()- and some other -egen- functions too.

The reference I gave earlier covers this kind of thing too, wanting to relate values to base dates.




Nick
[email protected]


On 16 August 2013 19:33, Nickolas Lyell <[email protected]> wrote:
> Great, this is really helpful.
>
> Now it would be great if there was a way to grab a year of a value for 
> calculating growth rates/
>
> I'm looking for something that would let  me say (using [] notation as if it were an array in javascript.  This makes sense to me, and I'm surprised if there is no such capability in STATA):
>
> (RGDP[y=peakRGDPyear] - RGDP[1990])/ RGDP[1990]
>
> As it is, I'm stuck creating a variable containing the data I want within each year, doing my calculations, and then dropping them after I have my growth rates.
>
> Nicholas Lyell
>
>
> -----Original Message-----
> From: [email protected] 
> [mailto:[email protected]] On Behalf Of Nick Cox
> Sent: Friday, August 16, 2013 1:28 PM
> To: [email protected]
> Subject: Re: st: FW: Max/Min and Growth rates among subsample(by year) 
> of panel data
>
> Here is a trick which I like.
>
> bysort fips_num: egen peakRGDP = max(RGDP) if year >2001 & year <2010 
> bysort fips_num (peakRGDP): replace peakRGDP = peakRGDP[1]
>
> can be put in one line
>
> bysort fips_num: egen peakRGDP = max(RGDP / (year >2001 & year <2010))
>
> Here's how it works:
>
> (year >2001 & year <2010) is true or false, evaluated as 1 when true and 0 when false.
>
> RGDP / (year >2001 & year <2010)  is thus RGDP/1 = RGDP or RGDP/0 = missing.
>
> Normally, dividing by 0 is something you just know not to do, but here it is totally deliberate: you know the result will be missing, but as usual Stata will just ignore the missings, which is exactly what you want.
>
> max(RGDP / (year >2001 & year <2010))
>
> therefore acts as if it were a conditional. You get a maximum, but 
> conditionally
>
> It's purely psychology, and accidental similarity of notation, but you will know | as indicating a conditional, and here it's just a little tired and leaning over slightly and is shown as /.
>
> There are other ways to do it. Here's one:
>
> bysort fips_num: egen peakRGDP = max(cond(year >2001 & year <2010, 
> RGDP, .))
>
> The -max()- function of -egen- takes an expression, which gives extra scope here.
>
> (You could also use -twithin()-, by the way.)
>
> As for when the peak or trough occurs, that's the same trick
>
> bysort fips_num: egen whenpeak = min(year / (RGDP == peakRGDP))
>
> Note that this code copes with ties too. If two or more years tie for 
> peak, it takes the first. If you want another definition, go for
> -max()- or whatever else makes sense.
>
> I am bailing out here, but I think some of this carries over to the rest of your problem.
>
> Footnotes: I see no value in a -reshape- here.
>
> The divide by zero trick is written up in
> http://www.stata-journal.com/article.html?article=dm0055
>
> Nick
> [email protected]
>
>
> On 16 August 2013 17:31, Nickolas Lyell <[email protected]> wrote:
>
>> I've been trying to analyze a large database of counties and their economic indicators from 1990-2013.
>>
>> I have it formatted long by fips number (a unique identifier for each US county) and year, with the indicators wide.
>>
>> I am trying to calculate several indicators within a subsample of years by county.  For instance, I would like to know the peak pre-recession level of real GDP (RGDP).  Since I don't want the 90's or post-recession data to show up, I would like to limit the maximum to 2002-2009.
>>
>> To get this value and its year as variables in the dataset, I did the following:
>>
>> sort fips_num year
>> by fips_num: egen peakRGDP = max(RGDP) if year >2001 & year <2010 
>> bysort fips_num (peakRGDP): replace peakRGDP = peakRGDP[1] by
>> fips_num: gen peakRGDPyear = year if peakRGDP==RGDP bysort fips_num
>> (peakRGDPyear): replace peakRGDPyear = peakRGDPyear[1]
>>
>> and the same thing for the trough value:
>>
>> sort fips_num year
>> by fips_num: egen troughRGDP = min(RGDP) if year >peakRGDPyear bysort 
>> fips_num (troughRGDP): replace troughRGDP = troughRGDP[1] by fips_num:
>> gen troughRGDPyear = year if troughRGDP==RGDP bysort fips_num
>> (troughRGDPyear): replace troughRGDPyear = troughRGDPyear[1]
>>
>> which works, but feels clunky.  And neither does it help me to calculate annualized growth rates among subsamples of years.  I would like to find out, for instance the annualized growth rate of RGDP from 1990-peakRGDPyear, peakRGDPyear-troughRGDPyear, troughRGDPyear-2013.
>>
>> Is there a better way to do this?  Maybe I need to convert my data back to wide with respect to year and use rowmax and rowmin functions, but that also seems clunky.
>
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