Re: st: RE: Dividing data into quintiles

[Clarice Martins <martins.clarice@gmail.com>]

Dear Nick,

You're correct. Forgive me.
At closer examination (and you guidance), I do understand what you mean.

THanks...!!! 

Clarice


On Oct 28, 2013, at 10:48 PM, Nick Cox wrote:

> You should look at the code more carefully to understand these comments.
> 
> 1. In David's original code, a new temporary variable was created to
> receive results from -xtile- each time round the loop. Once the loop
> is left these temporary variables were all still in memory.
> 
> In my code, I create a new variable each time around the loop but
> -drop- it as soon as the useful results it contains are transferred to
> the variable -quintile-. Once the loop is finished, there are no
> unnecessary variables still in memory.
> 
> Your suggestion "wouldn't it be good to have this temporary variable
> with signals for me each quintile?" misses this point and asks
> something quite different. The point is that the non-missing content
> of the variables created during the loop is copied across to the
> variable -quintile- in a correct version of the code. So, all the
> information has been preserved and nothing is thrown away that is not
> useless.
> 
> You only want the one variable, but we build up its contents in a loop.
> 
> 2. David suggested (the equivalent of)
> 
>   replace quintile = work
> 
> but this is buggy and should be
> 
>   replace quintile = work if rtype=="formation" & yrmonth == "`lev'"
> 
> as otherwise the missing values present for observations in which it
> is _not_ true that
> 
> rtype=="formation" & yrmonth == "`lev'"
> 
> will be transferred to -quintile- each time around the loop. The
> result will be that only the non-missing results from the last time
> around the loop would be saved (not quite a variable with entirely
> missing values, as you appeared to report earlier).
> 
> Nick
> njcoxstata@gmail.com
> 
> 
> On 28 October 2013 22:09, Clarice Martins <martins.clarice@gmail.com> wrote:
>> Dear Nick,
>> 
>> I do not see the problem with:
>> 
>>> I would not create a new temporary variable each
>>> time round the loop.
>> 
>> if i do need to know these quintiles for further calculation, wouldn't it be good to have this temporary variable with signals for me each quintile? What am I missing?
>> 
>>> The -replace- should be restricted to the
>>> observations it was calculated for;
>> 
>> this one honestly, i didn't understand what it does...
> 
> On Oct 28, 2013, at 7:49 PM, Nick Cox wrote:
> 
>>> Here is a tweak of David's code. I have not tested this, but I can see
>>> two problems with his code. The -replace- should be restricted to the
>>> observations it was calculated for; otherwise you overwrite good
>>> values with missings. I would not create a new temporary variable each
>>> time round the loop.
>>> 
>>> gen quintile = .
>>> quietly levelsof yrmonth, local(levs)
>>> 
>>> quietly foreach lev of local levs {
>>>        xtile work = return if rtype=="formation" & yrmonth == "`lev'", n(5)
>>>        replace quintile = work if rtype=="formation" & yrmonth == "`lev'"
>>>        drop work
>>> }
> 
> On 28 October 2013 21:33, Clarice Martins <martins.clarice@gmail.com> wrote:
> 
>>>> I believe you understood correctly!  Thank you for your suggestion!
>>>> 
>>>> But forgive me, if I am totally off, I am very new with Stata: I think I understood your code, but after creating the variable quintile, this one was kept empty. Should I see values recorded on that? Or the results of the code are stored virtually?   (sorry! very basic question!)
>>>> 
>>>> In my research, the next step will use returns for the companies in the 1st and 5th quintile of each  -rtype=="formation" & yrmonth == "`lev'"-  for calculations, so I do need to have available these quintiles (specifically 1st and 5th) for next step.
> 
> On Oct 28, 2013, at 4:58 PM, Radwin, David wrote:
> 
>>>>> If I understand correctly, you merely need to loop through each value of yrmonth and calculate quintiles for observations with that value for yrmonth. But you can't generate a new variable 151 times.
>>>>> 
>>>>> So you could do something this that creates quintiles for a temporary variable:
>>>>> 
>>>>> gen quintile = .
>>>>> quietly levelsof yrmonth, local(levs)
>>>>> foreach lev of local levs {
>>>>>     tempvar quint
>>>>>     xtile `quint' = return if rtype=="formation" & yrmonth == "`lev'", n(5)
>>>>>     replace quintile = `quint'
>>>>>     }
> 
> Clarice Martins
> 
>>>>>> I need to select a sub-group of my dataset and cut it into quintiles, in
>>>>>> order to proceed with calculations with top and bottom quintile.
>>>>>> 
>>>>>> - I use Stata 12.1 (for Mac)
>>>>>> 
>>>>>> - my data looks like this (now, after considerations from the group, thank
>>>>>> you!)
>>>>>> 
>>>>>> co_id ticker rtype   yrmonth return
>>>>>> 1 ABCB formation jun2000 0.01
>>>>>> 1 ABCB buysell jul2000 0.01
>>>>>> 1 ABCB holding ago2000 0.01
>>>>>> 2 AEDU formation jun2000 0.01
>>>>>> 2 AEDU buysell jul2000 0.01
>>>>>> 2 AEDU holding ago2000 0.01
>>>>>> 3 AMBV formation jun2000 0.01
>>>>>> 3 AMBV buysell jul2000 0.01
>>>>>> 3 AMBV holding ago2000 0.01
>>>>>> 
>>>>>> - i tryed both -egen cut- and -xtile-
>>>>>> 
>>>>>> egen quintile = cut(return) if rtype=="formation", group(5)
>>>>>> 
>>>>>> xtile quintile=return if rtype=="formation", n(5)
>>>>>> 
>>>>>> - when I use -if rtype=="formation"- both commands work fine!
>>>>>> 
>>>>>> - But, I need to select all observations rtype=="formation"  AND
>>>>>> yrmonth=="jun2000", then cut this subgroup into quintiles
>>>>>> 
>>>>>> - Then, I need to proceed with this filter/select and cut, for every
>>>>>> yrmonth (151 periods) on my data set
>>>>>> 
> 
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