Re: st: drop observations - if statement

[Nick Cox <njcoxstata@gmail.com>]

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)"?
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