KnockoutJS checked binding

HTML

<div data-bind="foreach: items">
<input type="radio" name="test" data-bind="checkedValue: $data, checked: $root.chosenItem" />
<span data-bind="text: itemName"></span>
</div>
<div><p>You have selected: <span data-bind="text:JSON.stringify(chosenItem())"></span></p></div>
<button data-bind="click:print">Print</button>

JavaScript

var ViewModel = {    
    items : ko.observableArray([
    { itemName: 'Choice 1' },
    { itemName: 'Choice 2' }
    ]),
    chosenItem : ko.observable()
};

ko.applyBindings(ViewModel);