title: Inference about Slope coefficient in R
body: <p>By default <code>lm</code> summary test slope coefficient equal to zero. My question is very basic. I want to know how to test slope coefficient equal to non-zero value. One approach could be to use <code>confint</code> but this does not provide p-value. I also wonder how to do one-sided test with <code>lm</code>.</p>
  
  <pre><code>ctl &lt;- c(4.17,5.58,5.18,6.11,4.50,4.61,5.17,4.53,5.33,5.14)
  trt &lt;- c(4.81,4.17,4.41,3.59,5.87,3.83,6.03,4.89,4.32,4.69)
  group &lt;- gl(2,10,20, labels=c("Ctl","Trt"))
  weight &lt;- c(ctl, trt)
  lm.D9 &lt;- lm(weight ~ group)
  summary(lm.D9)
  
  Call:
  lm(formula = weight ~ group)
  
  Residuals:
      Min      1Q  Median      3Q     Max 
  -1.0710 -0.4938  0.0685  0.2462  1.3690 
  
  Coefficients:
              Estimate Std. Error t value Pr(&gt;|t|)    
  (Intercept)   5.0320     0.2202  22.850 9.55e-15 ***
  groupTrt     -0.3710     0.3114  -1.191    0.249    
  ---
  Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 
  
  Residual standard error: 0.6964 on 18 degrees of freedom
  Multiple R-squared: 0.07308,    Adjusted R-squared: 0.02158 
  F-statistic: 1.419 on 1 and 18 DF,  p-value: 0.249 
  
  
  confint(lm.D9)
                2.5 %    97.5 %
  (Intercept)  4.56934 5.4946602
  groupTrt    -1.02530 0.2833003
  </code></pre>
  
  <p>Thanks for your time and effort.</p>