Afternoon/Morning/Evening all!!

I am currently using some very basic javascript as below to validate form fields:

HTML Code:
<script type="text/javascript">
function validate() { 
if (document.demo.field_1.value.length < 1) { 
alert("Please complete question 1"); 
return false; 
}
if (document.demo.field_2.selectedIndex == 0 ) { 
alert("Please complete question 2"); 
return false; 
}
return true; 
}
</script>
What I am hoping to do is have the same type of validation on some fields dependant on the value of a drop down. For example if question 3 = “Yes” then validate questions 4 – 7 but if question 3 = “No” then do not validate questions 4 – 7.
If anyone can help me with some javascript that would accomplish this I would be very grateful!

Many thanks

Legs