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st: RE: Re: Weighted number of observations
This isn't a -tabstat- issue as such. -tabstat-
just passes the buck to -summarize-, which
behaves in the same way.
The issue is delicate, but hinges, I surmise,
on this distinction. When -summarize- (e.g.) _uses_
the sum of the weights, it rescales first.
As quoted, [U] 14.1.6 includes the expression
"when it uses them", which thus appears not
ornamental, but crucial.
When -summarize- _displays_ the sum of the
weights, it displays the unscaled sum.
> Dear Toyoto,
> The auto data has 22 observations with foreign=1, not 50950. In [R]
> tabstat we read: "aweights and fweights are allowed; see [U] 14.1.6
> weight." [U] 14.1.6 states that most Stata commands rescale the
> aweights to sum to N. However, -tabstat- does not rescale the
> weights. The fact that -tabstat- treats aweights the same way as
> fweights is not clearly documented. One could also argue that this
> behavior is inconsistent.
> Friedrich Huebler
> --- Toyoto Iwata <firstname.lastname@example.org> wrote:
> > Dear Friedrich Huebler
> > You wrote about
> > > . tabstat foreign [aw=weight], stat(sum)
> > >
> > > variable | sum
> > > -------------+----------
> > > foreign | 50950
> > > ------------------------
> > Perhaps I miss the point, but,
> > .gen eachmean = foreign*weight
> > .gen sumeachmean = sum(eachmean) /* I don't know the mean of this.
> > */
> > . list sumeachmean in l
> > +----------+
> > | sumeac~n |
> > |----------|
> > 74. | 50950 |
> > +----------+
> > This seems to agree with the definition of the aweight.
> > [Online help says,]
> > aweights, or analytic weights, are weights that are inversely
> > proportional to the variance of an observation; i.e.,
> > the variance of the j-th observation is assumed to be sigma^2/w_j,
> > where w_j are the weights.
> > Typically, the observations represent averages and the weights are
> > the number of elements that gave rise to the average.
> > > The Stata User's Guide states in section 14.1.6: "For most Stata
> > > commands, the recorded scale of aweights is irrelevant; Stata
> > > internally rescales them to sum to N, the number of observations
> > in
> > > your data, when it uses them." It would be useful if the Stata
> > > documentation could make clear which commands don't use aweights
> > in this manner.
> > >
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