Re: st: drop observations - if statement

[Nick Cox <njcoxstata@gmail.com>]

For completeness (and correcting slightly), note that

dropmiss cusip, piasm trim obs

is equivalent to

drop if missing(cusip) | trim(cusip) == "."

There is no advantage to using -dropmiss- in this instance. The
advantage of using -dropmiss- is more evident in (e.g.) dropping all
variables that are all missing, which is just

dropmiss, force

-dropmiss- is from SJ.


Nick
njcoxstata@gmail.com


On 6 March 2014 15:43, Nick Cox <njcoxstata@gmail.com> wrote:
> This really shouldn't be difficult. Also, I've failed as a program
> author if my help is not clear.
>
> It is also crucial that you don't do anything destructive to a dataset
> without understanding exactly what you are doing.
>
> As established at length in this thread
>
> dropmiss cusip
>
> will _not_ do what you want because "." does not qualify as string
> missing to Stata. So, it appears that what you want is
>
> dropmiss cusip, piasm
>
> but
>
> drop if cusip == "."
>
> is an exact equivalent.
>
> Nick
> njcoxstata@gmail.com
>
>
> On 6 March 2014 15:35, R Zhang <r05zhang@gmail.com> wrote:
>> I need to drop those missing observations for my research goal.
>>
>> I typed findit dropmiss, and got SJ8-4 dm89_1.  and installed it.
>>
>> would it be dropmiss(cusip)?
>>
>> thanks,
>>
>>
>>
>> On Thu, Mar 6, 2014 at 10:19 AM, Nick Cox <njcoxstata@gmail.com> wrote:
>>> I can't answer that for you.
>>>
>>> It's not necessarily a good idea to -drop- observations just because
>>> some values are missing. That depends on your research goals, and I
>>> have no idea what they are. Also, missing values do little harm,
>>> because Stata usually ignores them.
>>>
>>> It's a conditional: if you want to -drop- data with missings,
>>> -dropmiss- (SJ) is just one tool. But it is, necessarily, no more than
>>> a wrapper for -drop-, so whatever it does you can do by other means.
>>>
>>>
>>> Nick
>>> njcoxstata@gmail.com
>>>
>>>
>>> On 6 March 2014 15:11, R Zhang <r05zhang@gmail.com> wrote:
>>>> Nick,
>>>>
>>>> thanks so much!   I ran di missing("."), it display 0, so . is not missing.
>>>>
>>>> should I still use the user written command you suggested to drop observations?
>>>>
>>>> -Rochelle
>>>>
>>>> On Thu, Mar 6, 2014 at 6:52 AM, Nick Cox <njcoxstata@gmail.com> wrote:
>>>>> Note also that in general Stata is highly literal when dealing with
>>>>> strings. Indeed it could hardly be anything else.
>>>>>
>>>>> So, when you see instances of "." that you want to -drop- but you ask
>>>>> that Stata -drop- instances of "", your command cannot have the
>>>>> desired effect.
>>>>>
>>>>> The user-written command -dropmiss- (Stata Journal) has an option
>>>>> -piasm-, comprehensible as "period is also string missing" for the
>>>>> convenience of users who wish to regard "." as missing. The existence
>>>>> of this option is evidence for the general rule, not against it!
>>>>>
>>>>> Nick
>>>>> njcoxstata@gmail.com
>>>>>
>>>>>
>>>>> On 6 March 2014 10:35, Nick Cox <njcoxstata@gmail.com> wrote:
>>>>>> "." may seem to mean missing in two possible senses;
>>>>>>
>>>>>> 1. Users may employ that coding for missing, but that's a personal
>>>>>> choice. Stata pays no special attention.
>>>>>>
>>>>>> 2. Stata will return numeric missing to -real(".")-
>>>>>>
>>>>>> . di real(".")
>>>>>>
>>>>>> .
>>>>>>
>>>>>> Notice that Stata displayed a period or stop, meaning numeric (often
>>>>>> called system) missing.
>>>>>>
>>>>>> But it does that to anything that can't be translated to a number:
>>>>>>
>>>>>> . di real("Stata")
>>>>>> .
>>>>>>
>>>>>> Is "." missing? Stata will answer this question for you
>>>>>>
>>>>>> . di missing(".")
>>>>>> 0
>>>>>>
>>>>>> 0 means false.
>>>>>>
>>>>>> Nick
>>>>>> njcoxstata@gmail.com
>>>>>>
>>>>>>
>>>>>> On 6 March 2014 03:03, R Zhang <r05zhang@gmail.com> wrote:
>>>>>>> thank you Jeph!
>>>>>>>
>>>>>>> I did "count if missing(cusip)", it shows zero observation. but when
>>>>>>> the data is openned in browse mode, cusip field is displayed as "." ,
>>>>>>> is that not missing?
>>>>>>>
>>>>>>> On Wed, Mar 5, 2014 at 6:00 PM, Jeph Herrin <info@flyingbuttress.net> wrote:
>>>>>>>> Are you sure any are missing?
>>>>>>>>
>>>>>>>> Try
>>>>>>>>
>>>>>>>>  count if missing(cusip)
>>>>>>>>
>>>>>>>> and see what you get.
>>>>>>>>
>>>>>>>> hth,
>>>>>>>> Jeph
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On 3/5/2014 5:41 PM, R Zhang wrote:
>>>>>>>>>
>>>>>>>>> Hi all,
>>>>>>>>>
>>>>>>>>> I used the following commands to drop observations based on a string
>>>>>>>>> variable - CUSIP- that takes missing values, i.e. delete observations
>>>>>>>>> with missing CUSIP,
>>>>>>>>>
>>>>>>>>> drop if cusip==""
>>>>>>>>>
>>>>>>>>> cusip is str10
>>>>>>>>>
>>>>>>>>> (0 observations deleted)
>>>>>>>>>
>>>>>>>>> could you explain why "(0 observations deleted)"?
>>> *
>>> *   For searches and help try:
>>> *   http://www.stata.com/help.cgi?search
>>> *   http://www.stata.com/support/faqs/resources/statalist-faq/
>>> *   http://www.ats.ucla.edu/stat/stata/
>> *
>> *   For searches and help try:
>> *   http://www.stata.com/help.cgi?search
>> *   http://www.stata.com/support/faqs/resources/statalist-faq/
>> *   http://www.ats.ucla.edu/stat/stata/
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/faqs/resources/statalist-faq/
*   http://www.ats.ucla.edu/stat/stata/