# st: RE: logit and LOESS graphs

 From "Nick Cox" To Subject st: RE: logit and LOESS graphs Date Fri, 7 Mar 2008 19:54:41 -0000

1. What was first called lowess was later called loess (and is also
implemented in other languages
as locfit). Stata has stuck to the name -lowess- for some time.

No matter, in Stata

. search loess

would have pointed Chris to helpful documentation. The name -loess- is
at least for some users most
unfortunate as it refers to a particular kind of deposit much studied by
sedimentologists and geomorphologists
(and not a smooth surface as Bill Cleveland claims somewhere).

No matter!

2. A logit curve, depending quite what is meant by that, can be obtained
pure and simple by

twoway function logit = logit(x)

twoway function inverse_logit = invlogit(x), ra(-5 5)

3. If you install -regplot- (-findit regplot- for locations), then
another way to get part of Maarten's
example is

logit foreign mpg
regplot

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

Maarten buis

--- Chris wrote:
> 1. Does anyone know how to perform LOESS in Stata?  I know how to do

> this in R but I'd prefer to avoid that, because R is memory
> inefficient.
>
> 2. Is there a way to display a logit curve?

Given the combination of question I am assuming you are looking for
something like the example below. However, there ares some weird jumps
in the lowess curve that I cannot explain.

-- Maarten

*----------- begin example ------------------
sysuse auto, clear
logit foreign mpg
sort mpg
predict logit, xb
predict pr, pr

twoway lowess foreign mpg || ///
line pr mpg, name(pr)

twoway lowess foreign mpg, logit || ///
line logit mpg, name(logit)
*----------------- end example -----------------

*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/