JavaScript validation depending on dropdown selection

Is it possible to execute a validate function (that runs at the form submit) only when a certain dropdown value is selected? For instance, if a user selects value ‘1’ from a dropdown on the form, a validate function will execute at submit. But if values ‘1’, ‘2’, or ‘3’ are selected from the same dropdown, upon submittal the validate function will NOT execute. Can I do this?

Subject: JavaScript validation depending on dropdown selection

I found a solution that works.

if (f.aa4.selectedIndex==6) {

Validate();

}

With 6 being the 7th selection in my dropdown (aa4).