Calling a function thats in a parrent page

[i00]

ok - i have a web page with the following in it :

<script>
public function doStuff() {
bla bla
}
</script>

now what i want to do is call that function from a child page that is embeded within an iframe so heres how i have tried to call it:

<script>
parent.document.doStuff()
</script>

and also

<script>
parent.doStuff()
</script>

however none of these work - what am i doing wrong ?

Thanks in advance
Kris Bennett