Radio button value - javascript

Radio button value - javascript

I have this code:
<input type="radio" name="pay" id="payid" value="banktransfer"> Bank
Transfer <br/>
<input type="radio" name="pay" id="payid" value="paypal"> PayPal <br/> <br/>
I try:
var sample = $('#payid').val()
alert(sample);
Why I have still banktransfer? (regardless of the selected input)?