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The bigger issue is that if you have lots of ties, 
it is most unlikely that your problem really is
suitable for -ipolate-, or indeed any other 
interpolation method. In fact users applying 
-ipolate- are probably confused about what 
interpolation is for. 

In the case of -ipolate-, the averaging is documented 
in the manual at [D] ipolate. This therefore 
is one of many cases in which reliance on 
the on-line help would lead you to miss  
a notable detail of the process. Nevertheless, 
it would do no harm for a line to be added to 
the help file explaining this detail. 

It is open to anyone to clone -ipolate- and 
modify the clone. The conservative behaviour 
in which ties are not averaged can be achieved 
without any programming whatsoever: 

ipolate yvar xvar, gen(yvar2) 
replace yvar2 = yvar if !mi(yvar) 

-ipolate- is an official Stata command and 
outwith my control. However, I looked at my 
-cipolate- command for cubic interpolation 
on SSC, which behaves in exactly the same 
way in averaging ties. That is no surprise
as I stole the -ipolate- code and made the minimum changes
necessary. I am not inclined to change the code
but I will add a word of explanation to the help 
file. 

Nick 
n.j.cox@durham.ac.uk 

austin nichols
 
> While I can see Nick's point about interpolation in general, I agree
> with Ben Jann that -ipolate- should not "replace" nonmissing values of
> the y variable when it interpolates using multiple y values per value
> of x, since according to its own help file it does not.  The missing
> value of rep78 for mpg==19 should be filled in, but not the four
> nonmissing values of rep78 for mpg==15 or 17.
> 
> I would add the following to  line 75 of ipolate.ado 
> (currently a blank line):
>    qui replace `z'=`usery' if `usery'<.
> and alter the first two lines to read
>   *! version 1.0 6sep2005 based on ipolate, version 1.3.3  21sep2004
>   program define nipolate, byable(onecall) sort
> then save the revised program as nipolate.ado, which produces
> 
>     rep78   mpg   rep78i  
>         .    14        .  
>         4    15        4  
>         3    15        3  
>         3    16        3  
>         5    17        5  
>         2    17        2  
>         3    18        3  
>         3    19        3  
>         .    19        3  
>         3    19        3  
>         3    21        3  
>         .    22      3.5  
>         4    23        4  
>         4    25        4  
>         4    25        4  
>         .    26      4.1  
>         .    26      4.1  
>         5    35        5  
> 
> in Ben's example.
> 
> Alternatively, you could add an option to line 5, making it e.g.
> 		*/ [ BY(varlist) Epolate noavg]
> and make  line 75:
>    if "`avg'"!="" { qui replace `z'=`usery' if `usery'<. }
> so nipolate would behave as does ipolate unless you specify noavg.
> 
> 
> On 9/5/05, Nick Cox <n.j.cox@durham.ac.uk> wrote:
> > -ipolate- interpolates linearly within gaps. That
> > is, it is assumed that the y variable varies linearly with
> > the x variable within any gaps. This is best seen
> > geometrically, as the last (x,y) pair before any gap
> > and the first (x,y) pair after any gap are just joined
> > by a straight line and intermediate results read
> > off directly.
> > 
> > In addition, a consequence of the assumption that
> > y is piecewise linear in x is
> > that repeated y's at any x are just averaged.
> > 
> > Plotting your results will make it easier to
> > see what is going on.
> > 
> > The help file is somewhat elliptical here.
> > 
> > Nick
> > n.j.cox@durham.ac.uk
> > 
> > > Ben Jann wrote:
> > > 
> > > Stata's -ipolate- command produces results I don't
> > > understand. Here is an example:
> > >
> > >       . set seed 2346
> > >       . sysuse auto
> > >       . drop if rep78<. & uniform()<.8
> > >       . ipolate rep78 mpg, g(rep78i)
> > >       . sort mpg
> > >       . list rep78 mpg rep78i, clean
> > >
> > >              rep78   mpg   rep78i
> > >         1.       .    14        .
> > >         2.       4    15      3.5
> > >         3.       3    15      3.5
> > >         4.       3    16        3
> > >         5.       2    17      3.5
> > >         6.       5    17      3.5
> > >         7.       3    18        3
> > >         8.       3    19        3
> > >         9.       3    19        3
> > >        10.       .    19        3
> > >        11.       3    21        3
> > >        12.       .    22      3.5
> > >        13.       4    23        4
> > >        14.       4    25        4
> > >        15.       4    25        4
> > >        16.       .    26      4.1
> > >        17.       .    26      4.1
> > >        18.       5    35        5
> > >
> > > -help ipolate- states that rep78i should equal rep78
> > > if rep78 is not missing ("ipolate creates newvar = yvar,
> > > where yvar is not missing"). This is certainly not the case
> > > in the above example. For some reason, "3.5" is stored
> > > for cases 2, 3, 5, and 6. Can someone explain this to me?

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