Hi Peter. Sorry for the delay in responding. Holidays and all.

So, you can have a perfectly valid formula give an error when evaluated in the Formula dialog if the input values cause it to try to compute an invalid value. For example, if your formula evaluates to some number divided by zero, you'll get an error. So even though the formula may be valid, the value it's trying to compute isn't.

For your formula `sum(scores)*(1/sqrt(count(scores)))`, if you don't enter values for the `scores` variable, then `count(scores)` evaluates to 0, and the square root of 0 is 0, and thus you end up with 1 divided by 0, which will give an error. Try filling in some values for the `scores` variable and see if you get a different result.