diff --git a/app/controllers/execution_environments_controller.rb b/app/controllers/execution_environments_controller.rb
index 574aaed7..eeae03f9 100644
--- a/app/controllers/execution_environments_controller.rb
+++ b/app/controllers/execution_environments_controller.rb
@@ -28,7 +28,42 @@ class ExecutionEnvironmentsController < ApplicationController
     render(json: @docker_client.execute_arbitrary_command(params[:command]))
   end
 
+
+  def working_time_query
+    """
+      SELECT exercise_id, avg(working_time) as average_time
+      FROM
+        (
+      SELECT user_id,
+             exercise_id,
+             sum(working_time_new) AS working_time
+      FROM
+        (SELECT user_id,
+                exercise_id,
+                CASE WHEN working_time >= '0:30:00' THEN '0' ELSE working_time END AS working_time_new
+         FROM
+            (SELECT user_id,
+                    exercise_id,
+                    id,
+                    (created_at - lag(created_at) over (PARTITION BY user_id, exercise_id
+                                                        ORDER BY created_at)) AS working_time
+            FROM submissions
+            WHERE exercise_id IN (SELECT ID FROM exercises WHERE execution_environment_id = #{@execution_environment.id})
+            GROUP BY exercise_id, user_id, id) AS foo) AS bar
+      GROUP BY user_id, exercise_id
+    ) AS baz GROUP BY exercise_id;
+    """
+  end
+
   def statistics
+    working_time_statistics = {}
+    ActiveRecord::Base.connection.execute(working_time_query).each do |tuple|
+      working_time_statistics[tuple["exercise_id"].to_i] = tuple
+    end
+
+    render locals: {
+      working_time_statistics: working_time_statistics
+    }
   end
 
   def execution_environment_params
diff --git a/app/policies/external_user_policy.rb b/app/policies/external_user_policy.rb
index 2e11060b..5932e3f0 100644
--- a/app/policies/external_user_policy.rb
+++ b/app/policies/external_user_policy.rb
@@ -1,5 +1,5 @@
 class ExternalUserPolicy < AdminOnlyPolicy
   def statistics?
-    admin?
+    admin? || author? || team_member?
   end
 end
diff --git a/app/views/execution_environments/statistics.html.slim b/app/views/execution_environments/statistics.html.slim
index 1b804e95..a0c99fe1 100644
--- a/app/views/execution_environments/statistics.html.slim
+++ b/app/views/execution_environments/statistics.html.slim
@@ -8,7 +8,6 @@ h1 = @execution_environment
           th.header = t(title)
     tbody
       - @execution_environment.exercises.each do |exercise|
-        - average_score = exercise.average_score
         tr
           td = link_to exercise.title, controller: "exercises", action: "statistics", id: exercise.id
           td = exercise.users.distinct.count
@@ -16,4 +15,4 @@ h1 = @execution_environment
           td = exercise.maximum_score
           td = 100 / exercise.maximum_score * average_score
           td = exercise.average_number_of_submissions
-          td = exercise.average_working_time
+          td = working_time_statistics[exercise.id]["average_time"]