[R] Possible bug in foreign library import of Stata datasets

[Paul Johnson]

Concerning this article, Christopher Zorn, "Generalized Estimating 
Equation Models for Correlated Data: A Review with Applications." 2001. 
American Journal of Political Science 45(April):470-90.

The author very kindly provides data for replication on his web page: 
http://www.emory.edu/POLS/zorn/Data/GEE.zip.

  I've been comparing the Professor Zorn's results obtained with Stata 
and R.  I ran into some trouble with the results in Table 2.  I traced 
the problem back to the R foreign library's data import.  Observe the 
variable "deml" in the Stata output:


table deml

----------------------
Lower of  |
two       |
POLITY    |
democracy |
s         |      Freq.
----------+-----------
    -10.00 |        826
     -9.00 |      3,829
     -8.00 |      2,161
     -7.00 |      6,847
     -6.00 |        541
     -5.00 |        451
     -4.00 |        152
     -3.00 |        306
     -2.00 |        145
     -1.00 |        252
      0.00 |         94
      1.00 |        103
      2.00 |        169
      3.00 |        108
      4.00 |        404
      5.00 |        634
      6.00 |        154
      7.00 |        281
      8.00 |        923
      9.00 |        258
     10.00 |      2,352
----------------------


The negative valued observations get mixed up in R:

 > library(foreign)
 > dat2 <- read.dta("table2.dta")
 > table(deml)
deml
    0    1    2    3    4    5    6    7    8    9   10  246  247
   94  103  169  108  404  634  154  281  923  258 2352  826 3829
  248  249  250 251  252  253  254  255
  2161 6847  541 451  152  306  145  252

The read.dta has translated the negative values as (256-deml).

Is this the kind of thing that is a bug, or have I missed something in 
the documentation about the handling of negative numbers?  Should a 
formal bug report be filed?


-- 
Paul E. Johnson                       email: pauljohn at ku.edu
Dept. of Political Science            http://lark.cc.ku.edu/~pauljohn
1541 Lilac Lane, Rm 504
University of Kansas                  Office: (785) 864-9086
Lawrence, Kansas 66044-3177           FAX: (785) 864-5700