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

 From Nick Cox To statalist@hsphsun2.harvard.edu Subject Re: st: Dataset of means from the three largest values of a group Date Mon, 28 Nov 2011 20:39:57 +0000

```Someone might want ideas on how to handle the missings assumed
previously not to exist.

Here's one way:

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:
>
>
> 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:
>> 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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```