What command should I use to make my program pause until the user presses a key?

[litdev]

In Small Basic, whether a variable is treated as number or array or string is context dependent.  The operator + works on all data types with precedence left to right.

Consider the following examples.

Code:

TextWindow.WriteLine(1+2)
TextWindow.WriteLine(1+2+"A")
TextWindow.WriteLine("A"+1+2)
TextWindow.WriteLine(Text.Append(1,2)+"A")

In your code I would also initialise SommeDesEcartsALaMoyenne to 0 before it is added to.  SB does assume variables are "" or 0 initially, but good practice to initialise something before you use it.  Also your SD calculation is unusual, usually it would be RootMeanSquare.