title: R returning zero / NULL coefficients with quantreg quantile regression package
body: <p>I am using the <code>quantreg</code> package to compute quantile regressions in R. I invoke my QR using the following command.</p>
  
  <pre><code>quantGsReg15 &lt;- rq(gsRMSD ~ kMeanGrp + medianDurationMS + flightHours + 
                     flightHoursType + landings30days + privPilot + afterKSS,
                     tau = .15 , data = accumulatedNM , method="fn")
  summary(quantGsReg15)
  </code></pre>
  
  <p>Now if I look at the return the coefficients for the variables flightHours and flightHoursType are zero/NULL.</p>
  
  <pre><code>tau: [1] 0.15
  
  Coefficients:
                   coefficients   lower bd       upper bd      
  (Intercept)        1.080000e-03  -1.381000e-02   1.618000e-02
  kMeanGrp2          1.510000e-03 -1.797693e+308   1.510000e-03
  kMeanGrp3          6.170000e-03  -2.630000e-03  1.797693e+308
  medianDurationMS  -1.000000e-05  -4.000000e-05   3.000000e-05
  flightHours        0.000000e+00   0.000000e+00   0.000000e+00
  flightHoursType    0.000000e+00   0.000000e+00   1.000000e-05
  landings30days     3.000000e-05  -6.600000e-04   3.600000e-04
  privPilotyes       4.440000e-03 -1.797693e+308   1.380000e-02
  afterKSS           3.000000e-05  -1.130000e-03   2.250000e-03
  </code></pre>
  
  <p>When adding <code>se = "nid"</code> I also get a warning message:</p>
  
  <blockquote>
    <p>Warning message:
        In summary.rq(quantGsReg15, se = "nid") : 7 non-positive fis</p>
  </blockquote>
  
  <p>An OLS regression I computed before indicates that this is most likely not be the case. I experimented with different QR methods ( method = "fn") but without success.</p>
  
  <p>Invoking the same commands for the 0.5 quantile returns a similar output. However, I get a different warning message:</p>
  
  <blockquote>
    <p>Warning message:
        In rq.fit.br(x, y, tau = tau, ci = TRUE, ...) : Solution may be nonunique</p>
  </blockquote>
  
  <p>What am I doing wrong? I googled for 30minutes now but without success.</p>