Add scales to framework. Change calculations to first group and then

average those groupings and the way up the framework.  Likert scores for
a survey_item are averaged.  Then all the survey_items in a scale are
averaged.  Then student scales in a measure are averaged.  And teacher
scales in a measure are averaged.  Then the average of those two
calculations becomes the score for a measure.  Then the measures in a
subcategory are averaged.

spec/models/scale_spec.rb

@@ -0,0 +1,41 @@
+require 'rails_helper'
+
+RSpec.describe Scale, type: :model do
+  let(:school) { create(:school) }
+  let(:academic_year) { create(:academic_year) }
+  let(:scale) { create(:scale) }
+
+  describe '.score' do
+    let(:teacher_survey_item_1) { create(:teacher_survey_item, scale:) }
+    let(:teacher_survey_item_2) { create(:teacher_survey_item, scale:) }
+    let(:teacher_survey_item_3) { create(:teacher_survey_item, scale:) }
+
+    before :each do
+      create(:survey_item_response,
+             survey_item: teacher_survey_item_1, academic_year:, school:, likert_score: 3)
+      create(:survey_item_response,
+             survey_item: teacher_survey_item_2, academic_year:, school:, likert_score: 4)
+      create(:survey_item_response,
+             survey_item: teacher_survey_item_3, academic_year:, school:, likert_score: 5)
+    end
+
+    it 'returns the average of the likert scores of the survey items' do
+      expect(scale.score(school:, academic_year:)).to eq 4
+    end
+
+    context 'when other scales exist' do
+      before :each do
+        create(:survey_item_response,
+               academic_year:, school:, likert_score: 1)
+        create(:survey_item_response,
+               academic_year:, school:, likert_score: 1)
+        create(:survey_item_response,
+               academic_year:, school:, likert_score: 1)
+      end
+
+      it 'does not affect the score for the original scale' do
+        expect(scale.score(school:, academic_year:)).to eq 4
+      end
+    end
+  end
+end