Angularjs Remover um objeto filho do objeto Json

Quando tento remover o objeto filho, estou recebendo um erro

TypeError: ctrl.otsact.tests.splice não é uma função

HTML:

<tr ng-repeat="act in ctrl.otsact.tests" ng-if="ctrl.editToggle">
    <td><span ng-click="ctrl.removeOTSACT(act.id)"> Delete </span></td>
</tr>

Script: estou processando splice para remover o objeto filho do formulário json

function removeOTSACT(index) {
    ctrl.otsact.tests.splice(index, 1);
}

Objeto Json

{  
   "ACT":{  
      "name":"ACT",
      "tests":{  
         "73":{  
            "id":73,
            "official_test_id":1,
            "student_id":165888,
            "test_date":"2017-05-12",
            "score":"0.0",
            "created_at":"2017-05-23 13:50:40",
            "created_by_id":2766,
            "updated_at":"2017-05-23 13:50:40",
            "updated_by_id":2766,
            "subjects":[  
               {  
                  "id":1,
                  "official_test_id":1,
                  "student_id":165888,
                  "official_test_subject_id":1,
                  "score":1,
                  "student_score_id":73,
                  "name":"English",
                  "is_consider":1
               },
               {  
                  "id":2,
                  "official_test_id":1,
                  "student_id":165888,
                  "official_test_subject_id":2,
                  "score":1,
                  "student_score_id":73,
                  "name":"Math",
                  "is_consider":1
               },
               {  
                  "id":3,
                  "official_test_id":1,
                  "student_id":165888,
                  "official_test_subject_id":3,
                  "score":1,
                  "student_score_id":73,
                  "name":"Reading",
                  "is_consider":1
               },
               {  
                  "id":4,
                  "official_test_id":1,
                  "student_id":165888,
                  "official_test_subject_id":4,
                  "score":1,
                  "student_score_id":73,
                  "name":"Science",
                  "is_consider":1
               },
               {  
                  "id":5,
                  "official_test_id":1,
                  "student_id":165888,
                  "official_test_subject_id":5,
                  "score":1,
                  "student_score_id":73,
                  "name":"Writing",
                  "is_consider":0
               }
            ]
         },
         "74":{  
            "id":74,
            "official_test_id":1,
            "student_id":165888,
            "test_date":"2017-05-12",
            "score":"0.0",
            "created_at":"2017-05-23 13:50:40",
            "created_by_id":2766,
            "updated_at":"2017-05-23 13:50:40",
            "updated_by_id":2766,
            "subjects":[  
               {  
                  "id":1,
                  "official_test_id":1,
                  "student_id":165888,
                  "official_test_subject_id":1,
                  "score":2,
                  "student_score_id":74,
                  "name":"English",
                  "is_consider":1
               },
               {  
                  "id":2,
                  "official_test_id":1,
                  "student_id":165888,
                  "official_test_subject_id":2,
                  "score":2,
                  "student_score_id":74,
                  "name":"Math",
                  "is_consider":1
               },
               {  
                  "id":3,
                  "official_test_id":1,
                  "student_id":165888,
                  "official_test_subject_id":3,
                  "score":2,
                  "student_score_id":74,
                  "name":"Reading",
                  "is_consider":1
               },
               {  
                  "id":4,
                  "official_test_id":1,
                  "student_id":165888,
                  "official_test_subject_id":4,
                  "score":2,
                  "student_score_id":74,
                  "name":"Science",
                  "is_consider":1
               },
               {  
                  "id":5,
                  "official_test_id":1,
                  "student_id":165888,
                  "official_test_subject_id":5,
                  "score":2,
                  "student_score_id":74,
                  "name":"Writing",
                  "is_consider":0
               }
            ]
         }
      }
   }
}

O que estou fazendo de errado no script acima

obrigado.

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