issue in checkbox of angular js

<div ng-repeat="n in mysize"
 <input type="checkbox" id="ckl_" ng-model="n[id].checked"
                                    ng-change="selectSizeEdit(p.size_arr,n.id)" ng-if="(p.size_arr.indexOf(n.id) > -1)" ng-checked="true"> 
</div>

In the above code the values inside p.size_arr in checkboxes will be checked. Bt i cant select new checkboxes other than in p.size_arr. Plz help me.