无法在角度中获取UI树中下拉列表的选定值

Question

HTML：

<select
    data-ng-model="completionPercentage"
    class="pull-right" 
    style="margin-right:20px" 
    data-ng-options="c.percentage for c in TaskCompletionPercentage" 
    ng-change="PercentageCompleted()">
  <option value="">0%</option>
</select>` 

Javascrpt：

$scope.PercentageCompleted = function () {
    alert($scope.completionPercentage);
}