Re: st: by prefix

[Richard Williams <richardwilliams.ndu@gmail.com>]

At 09:35 AM 6/4/2012, tashi lama wrote:

> Hello,
>
>   Here is sth I found Stata very unintelligent. A by prefix has to 
> be preceded by sort command although the dataset is already sorted 
> by design. Take a look at this
>
>
>
> var1    var2
>
> A         3
>
> A         4
>
> A         1
>
> B         2
>
> B         5
>
>
>
>
>
> The dataset is already sorted by var1 as you all can see. Now when I 
> apply by prefix
>
> by var1: summ var2    says not sorted.
>
> Stata obliges you to write
>
> bysort var1: summ var2    which I think is overkill. Any thoughts?

Yes, but Stata doesn't know it is already sorted. The data manual 
entry for -sort- says this:

"Stata does not track changes in the sort order and will sometimes 
decide that a dataset is not sorted when, in fact, it is. For 
instance, if we were to change the first observation of our auto 
dataset from 12 miles per gallon to 10, Stata would decide that the 
dataset is "Sorted by:" nothing, just as it did above when we changed 
mpg from 12 to 13. Our change in example 2 did change the order of 
the data, so Stata was correct. Changing mpg from 12 to 10, however, 
does not really affect the sort order.

"As far as Stata is concerned, any change to the variables on which 
the data are sorted means that the data are no longer sorted, even if 
the change actually leaves the order unchanged. Stata may be dumb, 
but it is also fast. It sorts already-sorted datasets instantly, so 
Stata's ignorance costs us little."


-------------------------------------------
Richard Williams, Notre Dame Dept of Sociology
OFFICE: (574)631-6668, (574)631-6463
HOME:   (574)289-5227
EMAIL:  Richard.A.Williams.5@ND.Edu
WWW:    http://www.nd.edu/~rwilliam

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