How to get out the selected value when using RadioButtonGroup in flex

<mx:FormItem label="Print:">
<mx:HBox verticalAlign="middle" horizontalGap="15">
<mx:RadioButtonGroup />
<mx:RadioButton label="Not printing" groupName="dayinID" selected="false"/>
<mx:RadioButton label="Print" groupName="dayinID"/>
</mx:HBox>
</mx:FormItem>