Paul Schellevis -

can someone explain what equation Excel use to calculate the t-toets? <

There are three different t-tests. As far as I know, Excel uses the same
formulas that you would find in any statistics textbook. And those formulas
are shown in Excel's built-in Help.

For example, to display the equations in Excel 2003, choose Help | Microsoft
Excel Help | Offline Help | "About Statistical Analysis Tools" | t_Test.

- Mike

www.mikemiddleton.com