title: Mathematica: Operations on list with missing values
body: <p>I try and perform operations on a set of lists.  For example:<br>
  Lists:
  (1, 1, 2, 3, 3, 4, 4, 4, 4, 4, 4, 5),
  (1, 1, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA),
  (1, 1, 1, 1, NA, NA, NA, NA, NA, NA, NA, NA),
  (1, 1, 2, 3, 3, 4, 4, 4, 4, 4, 4, 5)</p>
  
  <p>Then I would like for example to have a list containing the means.
  Mean[lists] doesnt run properly because it does not know how to handle the missing values "NA".
  What I expect is:
  (1, 1, 5/3, 7/3, 3, 4, 4, etc.)</p>
  
  <p>Does anybody how to ignore the missing or 'text' values? Like you would operate such a function in excel for example.
  Thanks! </p>