How can I get Excel to produce a random sample of data?

This procedure will highlight every 10th cell / row

1) Create a column nearby with the numbers 1 to the last number row you have. I'll assume this starts at cell A1

2) Highlight all the cells that you wish to have highlighted

3) Click Format > Conditional Formatting

4) Click on "Cell Value Is" and change to "Formula Is"

5) In the next field enter this formula:

=MOD($A1,10)=0

6) Click Format and choose your highlight (Font, Border, Patterns)

7) Click OK and OK

If you wish to, you can hide the column of numbers (1,2,3 etc) by clicking on any cell in the column and clicking Format > Column > Hide, but make sure there is nothing else important in the same column otherwise this will disappear as well.

Hope this helps