R accuracy

Hello,

I am trying to test the precision of R on datasets from The 
Statistical Reference Datasets Project 
http://www.itl.nist.gov/div898/strd/index.html and I don't manage to 
understand how R is storing its results.

For example, I calculate a mean on the michelso dataset (100 values) and find:

 m=mean(michel)
 m

      V1
299.8524

 print(m,digits=15)

      V1
299.8524

 print(m,digits=22)

                V1
299.85239999999993


The certified value of the mean is 299.85240, so I try

 print(m-299.8524)

           V1
-5.684342e-14

 print(m-299.8524,digits=15)

                   V1
-5.68434188608080e-14

Does it have a sens to print with more than 15 signifiant digits?
Why is the difference not equal to zero?

I am using R 2.0.1 under Windows XP.

Regards,

Anthony Landrevie


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