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Re: st: Dataset of means from the three largest values of a group


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Dataset of means from the three largest values of a group
Date   Tue, 29 Nov 2011 09:14:39 +0000

The emphasis in the original posting was on creating a new dataset.
Let' s underline that there are several ways to do that. Here's
another

drop if missing(trade)
bysort country year (trade) : gen trade1 = trade[_N]
by country year : gen trade2 = trade[_N-1]
by country year : gen trade3 = trade[_N-2]
by country year : egen meanhighest = rowmean(trade?)
by country year : keep if _n == 1

On Mon, Nov 28, 2011 at 8:39 PM, Nick Cox <njcoxstata@gmail.com> wrote:
> Someone might want ideas on how to handle the missings assumed
> previously not to exist.
>
> Here's one way:
>
> gen ismissing = missing(trade)
> bysort ismissing country year (trade)  :  gen tag  = (_N - _n) < 3
> by ismissing country year : egen meanhighest = mean(trade / tag)
> bysort country year (meanhighest) : replace meanhighest = meanhighest[1]
> drop ismissing
>
> Nick
>
> On Mon, Nov 28, 2011 at 8:25 PM, Nick Cox <njcoxstata@gmail.com> wrote:
>> I assume no missing values for -trade-. "3" here evidently means here "up to 3"
>>
>> bysort country year (trade)  :  gen tag  = (_N - _n) < 3
>>
>> by country year : egen meanhighest = mean(trade / tag)
>>
>> On why division by zero can be useful, see
>>
>> SJ-11-2 dm0055  . . . . . . . . . . . . . .  Speaking Stata: Compared with ...
>>        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N. J. Cox
>>        Q2/11   SJ 11(2):305--314                                (no commands)
>>        reviews techniques for relating values to values in other
>>        observations
>>
>> The second highest (silver medallist) is more robust-resistant to outliers:
>>
>> bysort country year (trade) : gen silver = trade[_N-1]
>>
>> Nick
>>
>> On Mon, Nov 28, 2011 at 8:03 PM, Iulian Ihnatov <iulian.ihnatov@uaic.ro> wrote:
>>
>>> I have the following dataset for the period of 1999 to 2010:
>>> country    year     partner     trade
>>> AFG          1999    USA          12345
>>> AFG          1999    DEU          9875
>>> AFG          1999    FRA           25487
>>> ........................
>>> AFG          2000    USA           5454
>>> AFG          2000    HUN          5454
>>> ........................
>>> HUN         1999    DEU           58744
>>> ........................
>>>
>>> I need to create a dataset of means of the "trade" variable, grouped by
>>> country and year, but only for the three largest observations of each group.
>>> I may use - collapse (mean) trade, by(country year) -, but I don't know how
>>> to isolate the largest three values from each group (in some years, there
>>> are only 1 or 2 observations available, in others more than 10). Any help
>>> would be highly appreciated.
>>
>

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