Dear Marco, thanks for your reply and I now have a related problem. Below is code that successfully reads 22 columns of data from data matrix trydta. The command Mean correctly calculates the mean for data in column 7, which has both missing and non-missing data values.

colmean = Mean[Select[trydta[[All, 7]], NumberQ]] // N

But I would like to get a table of means for all 22 variables. Do you know how I need to modify the command to tell mathematica that I would like means not only for column 7 but for columns 1 - 22?

Thanks again for your help.

Pat

As an alternative, you could do something like this:

in:=sampleData = 
  SemanticImport["ExampleData/elements.xls", Automatic, 
   "NamedColumns"];
in:=Select[#, NumberQ] & /@ Values[sampleData];
in:=Mean[#] & /@ %

out:={5., Mean[{}], Mean[{}], 10.3094}

This and the previous answer may be equally convenient if your goal is to get a single result for each consecutive column. But the resulting output will be formatted as a list, rather than a table.

On the other hand, if you want to operate on specific columns in an arbitrary order, you could also do this:

in:=cols = Keys[sampleData];
in:=KeyTake[sampleData, {cols[[1]], cols[[4]]}];
in:=Select[#, NumberQ] & /@ Values[%];
in:=Mean[#] & /@ %

out:={5., 10.3094}