title: TukeyHSD adjusted P value is 0.0000000
body: <p>I just performed a factorial ANOVA, followed by the <code>TukeyHSD</code> post-test.  Some of my adjusted P values from the <code>TukeyHSD</code> output are <code>0.0000000</code>.  Can these P values really be zero?  Or is this a rounding situation, and my true P value might be something like 1e-17, that is rounded to <code>0.0000000</code>.</p>
  
  <p>Are there any options for the <code>TukeyHSD()</code> function in R that will give output P-values that contain exponents?  </p>
  
  <p>Here is a snippet of my output:</p>
  
  <pre><code>TukeyHSD(fit)
  
    Tukey multiple comparisons of means
      95% family-wise confidence level
  
  Fit: aov(formula = lum ~ cells * treatment)
  
  $`cells:treatment`
                      diff         lwr          upr     p adj
  NULL:a-KR:a     -266.5833333 -337.887800 -195.2788663 0.0000000
  WT:a-KR:a       -196.3333333 -267.637800 -125.0288663 0.0000000
  KR:ar-KR:a        83.4166667   12.112200  154.7211337 0.0053485
  NULL:ar-KR:a    -283.5000000 -354.804467 -212.1955330 0.0000000
  WT:ar-KR:a      -196.7500000 -268.054467 -125.4455330 0.0000000
  KR:e-KR:a       -219.0833333 -290.387800 -147.7788663 0.0000000
  NULL:e-KR:a     -185.0833333 -256.387800 -113.7788663 0.0000000
  WT:e-KR:a        -96.1666667 -167.471134  -24.8621996 0.0003216
  </code></pre>