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Re: st: How to drop missing values by period?


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: How to drop missing values by period?
Date   Mon, 11 Feb 2013 16:22:12 +0000

Sorry, but this looks to me like the same question, and my answer is the same.

Please do try my code, or the equivalent, on a copy of your data. Note
that you don't -drop- in terms of identifier or period.

Nick

On Mon, Feb 11, 2013 at 4:16 PM, Xixi Lin <winnielxx@gmail.com> wrote:
> Hi Nick,
>
> I know what do you mean, if I, for instance, drop id 1, then in period
> 2, id 1 is also gone. I am thinking is that possible only to drop id 1
> in period 1, while in other periods, id 1 is still there. I am not
> sure did I clearify this time?
>
> Best,
> Xixi Lin
>
> On Mon, Feb 11, 2013 at 10:25 AM, Nick Cox <njcoxstata@gmail.com> wrote:
>> Thanks for the example. Stata usually takes care of this for you in that
>>
>> 1. A command uses as many as observations as possible.
>>
>> 2. Most statistical commands ignore missing values.
>>
>> But in your example
>>
>> . drop if missing(x1, x2)
>>
>> will drop observations if either -x1- or -x2- is missing. It's not
>> clear to me that you need to think of this period by period, as that
>> makes no difference to the outcome.
>>
>> Nick
>>
>> On Mon, Feb 11, 2013 at 3:18 PM, Xixi Lin <winnielxx@gmail.com> wrote:
>>> Hi Nick,
>>>
>>> Thanks for the instant reply. What I mean is like:
>>> id      period  x1      x2
>>> 1       1       1       4
>>> 2       1       2       .
>>> 3       1       .       2
>>> 1       2       4       4
>>> 2       2       5       3
>>> 3       2       .       2
>>>
>>> In period 1, I want to keep id 1 because id 2 and id 3 have missing
>>> values; in period 2, I want to keep id 1 &2, because id 3 has missing
>>> values.
>>>
>>> I have time series data, but I wanna do a cross-sectional regressions
>>> and wanna keep the maximum of observations. So I am wondering if it is
>>> possible to drop obs only in one period while keeping it in  another
>>> period.
>>>
>>> Thanks.
>>> Xixi Lin
>>>
>>> On Mon, Feb 11, 2013 at 9:59 AM, Nick Cox <njcoxstata@gmail.com> wrote:
>>>> This sounds to me as if you want -keep- and -drop- the same
>>>> observations. That can't be done.
>>>>
>>>> Perhaps you should show what your data look like with variable names,
>>>> and link your question to such an example.
>>>>
>>>> Nick
>>>>
>>>> On Mon, Feb 11, 2013 at 2:53 PM, Xixi Lin <winnielxx@gmail.com> wrote:
>>>>
>>>>> I tried to drop missing values period by period, in 120 periods. In other
>>>>> words, in period 1, I want to drop the observations if dependent variables
>>>>> have missing values only in period 1; while in period 2, drop the
>>>>> observations if dependent variables have missing values only in period 2,
>>>>> should keep those that missing in period 1 but not missing in period 2. For
>>>>> instance:
>>>>>
>>>>> In period 1, I want to keep id 1; in period 2, I want to keep id 1 &2.
>>>>>
>>>>> Does anyone knows how to do that? Thanks a lot.
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