title: linear model having 4 predictors
body: <p>I am trying to fit a linear model having 4 predictors. The problem I am facing is my code doesn't estimate the one parameter. Every time when I put the one variable at last of my lm formula it doesn't estimate it. My code is:</p>
  
  <pre><code>AllData  &lt;- read.csv("AllBandReflectance.csv",header = T)
  Swir2ref &lt;- AllData$band7 
  x1 &lt;- AllData$X1
  x2 &lt;- AllData$X2
  y1 &lt;- AllData$Y1
  y2 &lt;- AllData$Y2
  
  linear.model &lt;- lm( Swir2ref ~ x1 + y1 +x2 +y2 , data = AllData )
  summary(linear.model)
  
  Call:
  lm(formula = Swir2ref ~ x1 + y1 + x2 + y2, data = AllData)
  
  Residuals:
        Min        1Q    Median        3Q       Max 
  -0.027277 -0.008793 -0.000689  0.010085  0.035097 
  
  Coefficients: (1 not defined because of singularities)
              Estimate Std. Error t value Pr(&gt;|t|)    
  (Intercept) 0.595593   0.002006 296.964   &lt;2e-16 ***
  x1          0.002175   0.003462   0.628    0.532    
  y1          0.001498   0.003638   0.412    0.682    
  x2          0.022671   0.018786   1.207    0.232    
  y2                NA         NA      NA       NA    
  ---
  Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
  
  Residual standard error: 0.01437 on 67 degrees of freedom
  Multiple R-squared:  0.02876,   Adjusted R-squared:  -0.01473 
  F-statistic: 0.6613 on 3 and 67 DF,  p-value: 0.5787
  </code></pre>