Path: janda.org/c10 > Syllabus > Outline > Topics and Readings > Correlating Two Continuous Variables
Linear Regression > Dropping Washington, D.C.

The Effect of Dropping Washington, D.C. from the Analysis:
a smaller correlation
 

SPSS offers methods for selecting cases for analysis. We will learn them later. This simple syntax command "selects" only those cases (states) that voter greater than 15% in 1980. That excludes Washington, D.C. from the analysis.

select if (reagan80 gt 15). <--drops D.C. by selecting only states that voted over 15% fo Reagan)

                            Plot of REAGAN84 with REAGAN80
         ++----+----+----+----+----+----+----+----+----+----+----+----+----++
      100+                                                                  +
         |                                                                  |
         |                                                                  |
         |                                                                  R
         |                                                                  |
     87.5+                                                                  +
         |                                                                  |
P        |                                                                  |
C        |                                                                  |
T        |                                                                  |
       75+                                               1                  +
V        |                                           1                      |
O        |                                      11 1 1                      |
T        |                                   1  1  1                        |
E        |                                1   2  1  1                       |
     62.5+                                2 1 1  1                          +
F        |                           1  13431 21                            |
O        |                             1    1                               |
R        |                            111231                                |
         |                        1    1                                    |
R      50+                           11                                     +
E        |                                                                  |
A        |                                                                  |
G        |                                                                  |
A        |                                                                  |
N    37.5+                                                                  +
,        |                                                                  |
         |                                                                  |
1        |                                                                  |
9        |                                                                  |
8      25R                                                                  +
4        |                                                                  |
         |                                                                  |
         |                                                                  |
         |                                                                  |
     12.5+                                                                  +
         |                                                                  |
         |                                                                  |
         |                                                                  |
         |                                                                  |
        0+                                                                  +
         ++----+----+----+----+----+----+----+----+----+----+----+----+----++
          0  15.38462  30.76923  46.15385  61.53846  76.92308  92.30769
         
                              PCT VOTE FOR REAGAN, 1980
         
        50 cases plotted. Regression statistics of REAGAN84 on REAGAN80:
         
Correlation= .85       R-Squared= .73 Intercept = 25.8 Slope = .67
 

Note that, by dropping Washington from the analysis, the correlation drops from r=.90 to r=.85, a loss of .05.

However, the R-square drops from .81 to .73, a loss of .08.