Error Variation

I ran five identical runs of the same program on the same input:

       User+System Time = 1.252102 Seconds
       User+System Time = 1.260666 Seconds
       User+System Time = 1.280666 Seconds
       User+System Time = 1.319948 Seconds
       User+System Time = 1.309948 Seconds

That's more than 5% variation

     %Time ExclSec CumulS #Calls sec/call Csec/c  Name
      65.4   0.820  0.819    109   0.0075 0.0075  main::letter_histogram
      63.4   0.800  0.799    109   0.0073 0.0073  main::letter_histogram
      62.4   0.800  0.799    109   0.0073 0.0073  main::letter_histogram
      65.1   0.860  0.859    109   0.0079 0.0079  main::letter_histogram
      64.8   0.850  0.849    109   0.0078 0.0078  main::letter_histogram

Ditto
Conclusion: Don't put any faith in the exact numbers
Corollary: If someone tells you that X is 5% faster than Y, ignore them