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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 15:25:15 +0000

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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