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Re: st: drop observations - if statement
From
R Zhang <[email protected]>
To
[email protected]
Subject
Re: st: drop observations - if statement
Date
Thu, 6 Mar 2014 22:15:08 -0500
thank you, Roberto !!!
On Thu, Mar 6, 2014 at 4:35 PM, Roberto Ferrer <[email protected]> wrote:
> Rochelle,
>
> I would just add that if you're not completely sure of the values of
> your "missings", you can try the -trim()- function. For example,
> suppose that -cusip- were to take the value " ." (that's three
> blanks) AND " . " (two + three blanks). Then, your command would be
> leaving some "missings" behind.
>
> This bypasses that issue:
>
> drop if trim(cusip) == "."
>
> See -help trim-.
>
> On Thu, Mar 6, 2014 at 3:42 PM, R Zhang <[email protected]> wrote:
>> thanks again!
>>
>> I found out this statement works drop if cusip==" ." (leading
>> empty space , 1400 observations dropped)
>>
>> On Thu, Mar 6, 2014 at 10:43 AM, Nick Cox <[email protected]> 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
>>> [email protected]
>>>
>>>
>>> On 6 March 2014 15:35, R Zhang <[email protected]> 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 <[email protected]> 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
>>>>> [email protected]
>>>>>
>>>>>
>>>>> On 6 March 2014 15:11, R Zhang <[email protected]> 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 <[email protected]> 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
>>>>>>> [email protected]
>>>>>>>
>>>>>>>
>>>>>>> On 6 March 2014 10:35, Nick Cox <[email protected]> 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
>>>>>>>> [email protected]
>>>>>>>>
>>>>>>>>
>>>>>>>> On 6 March 2014 03:03, R Zhang <[email protected]> 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 <[email protected]> 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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