Date:         Thu, 4 Dec 2008 11:27:44 -0800
Reply-To:     zolotarov@GMAIL.COM
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         zolotarov@GMAIL.COM
Organization: http://groups.google.com
Subject:      Comparing two regression lines
Comments: To: sas-l@uga.edu
Content-Type: text/plain; charset=ISO-8859-1

Hello,

Every two weeks i've measured growth diameter of fungal colonies grown
in two different environmental conditions and i want to compare their
effect on fungal growth using linear regression comparison.

I've found two references that present different codes for comparison
of linear regression coefficients.

Example 1 (from http://udel.edu/~mcdonald/statancova.html)
data gators;
   input sex $ snoutvent pelvicwidth;
   cards;
male      1.10    7.62
male      1.19    8.20
male      1.13    8.00
male      1.15    9.60
male      0.96    6.50
male      1.19    8.17
male      1.06    7.20
male      0.70    4.65
male      0.70    5.04
male      1.04    8.83
male      1.15    8.01
male      1.10    6.84
male      1.15    8.37
male      1.15    7.36
male      0.91    6.43
male      1.45    9.43
male      1.22    7.70
male      1.33   10.20
male      1.38    9.14
female    1.24    7.64
female    1.02    6.31
female    0.93    5.90
female    0.71    4.48
female    1.03    6.03
female    1.02    6.60
female    0.95    5.88
female    1.03    6.77
female    0.96    6.47
female    1.16    7.56
female    0.93    6.13
female    1.04    6.76
female    1.03    6.63
female    0.93    5.93
female    0.85    6.52
female    1.23    9.23
;

proc glm data=gators;
   class sex;
   model pelvicwidth=snoutvent sex snoutvent*sex;
proc glm data=gators;
   class sex;
   model pelvicwidth=snoutvent sex;
   run;

and Example 2 (from http://www.ats.ucla.edu/stat/sas/faq/compreg2.htm)
DATA htwt;
  INPUT id Gender $ height weight ;
CARDS;
 1   F  56 117
 2   F  60 125
 3   F  64 133
 4   F  68 141
 5   F  72 149
 6   F  54 109
 7   F  62 128
 8   F  65 131
 9   F  65 131
10   F  70 145
11   M  64 211
12   M  68 223
13   M  72 235
14   M  76 247
15   M  80 259
16   M  62 201
17   M  69 228
18   M  74 245
19   M  75 241
20   M  82 269
;
RUN;

data htwt2;
  set htwt;

  female = . ;
  IF gender = "F" then female = 1;
  IF gender = "M" then female = 0;

  femht = female*height ;
PROC GLM DATA=htwt2 ;
  CLASS gender ;
  MODEL weight = gender height gender*height / SOLUTION ;
RUN;


The two codes give the same probability for slope comparison, but
different probabilities for intercepts, which is the correct one for
comparison of intercepts?

Thank you.