title: R update() interaction term not dropped
body: <p>Problem: I intend to fit a linear model with interaction terms. After estimating a "full" model, I want to remove the not significant interaction terms. However, after using the function update(lm(),.~.-interaction) on my model, nothing happens. Please help.</p>
  
  <p>Data:</p>
  
  <pre><code>library(car)
  data(Prestige)
  Prestige_compl &lt;- Prestige[complete.cases(Prestige),] #rm NA's
  attach(Prestige_compl)
  </code></pre>
  
  <p>Model:</p>
  
  <pre><code>modR0 &lt;- lm(prestige   ~
               income             +
               education          +
               women              +
               income    * type   +
               education * type   +
               women     * type   ,
               data      = Prestige_compl) 
  
  # fit a linear model with interaction terms
  
  summary.lm(modR0)
  
  Coefficients:
                       Estimate Std. Error t value Pr(&gt;|t|)    
  (Intercept)        -5.822e+00  7.311e+00  -0.796  0.42803    
  income              4.692e-03  6.691e-04   7.013 5.00e-10 ***
  education           1.625e+00  9.163e-01   1.773  0.07971 .  
  women               1.343e-01  4.656e-02   2.885  0.00494 ** 
  typeprof            2.436e+01  1.351e+01   1.803  0.07496 .  
  typewc             -2.178e+01  1.727e+01  -1.261  0.21081    
  income:typeprof    -4.144e-03  7.132e-04  -5.810 1.03e-07 ***
  income:typewc      -7.527e-04  1.814e-03  -0.415  0.67924    
  education:typeprof  1.512e+00  1.235e+00   1.224  0.22423    
  education:typewc    2.123e+00  2.190e+00   0.970  0.33491    
  women:typeprof     -1.601e-01  6.506e-02  -2.460  0.01588 *  
  women:typewc        2.893e-02  1.117e-01   0.259  0.79619    
  </code></pre>
  
  <p>Rm not significant interaction terms:</p>
  
  <pre><code>modR1 &lt;- update(modR0, .~. -women:typewc)
  summary.lm(modR1)
  
  Coefficients:
                       Estimate Std. Error t value Pr(&gt;|t|)    
  (Intercept)        -5.822e+00  7.311e+00  -0.796  0.42803    
  income              4.692e-03  6.691e-04   7.013 5.00e-10 ***
  education           1.625e+00  9.163e-01   1.773  0.07971 .  
  women               1.343e-01  4.656e-02   2.885  0.00494 ** 
  typeprof            2.436e+01  1.351e+01   1.803  0.07496 .  
  typewc             -2.178e+01  1.727e+01  -1.261  0.21081    
  income:typeprof    -4.144e-03  7.132e-04  -5.810 1.03e-07 ***
  income:typewc      -7.527e-04  1.814e-03  -0.415  0.67924    
  education:typeprof  1.512e+00  1.235e+00   1.224  0.22423    
  education:typewc    2.123e+00  2.190e+00   0.970  0.33491    
  women:typeprof     -1.601e-01  6.506e-02  -2.460  0.01588 *  
  women:typewc        2.893e-02  1.117e-01   0.259  0.79619  
  </code></pre>
  
  <p>Why is the interaction term that should be removed still there?</p>