Here is an example of how to use a shift register to build an array on the fly inside a loop. Use the output of your Mean.vi to replace the random number generator in this example. Don't forget to initialize the shift register before the loop as shown in the example.
Thanks for your suggestion. Unfortunately I�ve got Labview 7 and cant open the example you posted. I get an error message say i need version 7.1 to open the file