st: _rmcoll issues

[Stas Kolenikov <skolenik@gmail.com>]

I have the following cross-tab of two variables:

. tab pd2usmex years_in_us

 PD2. What |
   country |
  were you |                 years_in_us
  born in? |         1          2          3          4 |     Total
-----------+--------------------------------------------+----------
         1 |       240          0          0          0 |       240
         2 |         0        103        451        378 |       932
         9 |         0         33         99        123 |       255
-----------+--------------------------------------------+----------
     Total |       240        136        550        501 |     1,427

For whatever reason, _rmcoll senses that something is fishy between
the two variables, but decides to drop the last category of the second
variable, rather than the first one on which these two variables
logically overlap.

. _rmcoll ib1.pd2usmex ib1.years_in_us
note: 4.years_in_us omitted because of collinearity

I think this is a bit odd.

-- Stas Kolenikov, PhD, PStat (ASA, SSC)
-- Senior Survey Statistician, Abt SRBI
-- Opinions stated in this email are mine only, and do not reflect the
position of my employer
-- http://stas.kolenikov.name
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