diff --git a/app/controllers/exercises_controller.rb b/app/controllers/exercises_controller.rb
index 8855228e..b506afdf 100644
--- a/app/controllers/exercises_controller.rb
+++ b/app/controllers/exercises_controller.rb
@@ -3,6 +3,7 @@ class ExercisesController < ApplicationController
   include Lti
   include SubmissionParameters
   include SubmissionScoring
+  include TimeHelper
 
   before_action :handle_file_uploads, only: [:create, :update]
   before_action :set_execution_environments, only: [:create, :edit, :new, :update]
@@ -347,14 +348,15 @@ class ExercisesController < ApplicationController
   def statistics
     if(@external_user)
       @submissions = Submission.where("user_id = ?  AND exercise_id = ?", @external_user.id, @exercise.id).order("created_at")
-      @submissions_and_interventions = (@submissions + UserExerciseIntervention.where("user_id = ?  AND exercise_id = ?", @external_user.id, @exercise.id)).sort_by { |a| a.created_at }
-      deltas = @submissions.map.with_index do |item, index|
-        delta = item.created_at - @submissions[index - 1].created_at if index > 0
+      interventions = UserExerciseIntervention.where("user_id = ?  AND exercise_id = ?", @external_user.id, @exercise.id)
+      @all_events = (@submissions + interventions).sort_by { |a| a.created_at }
+      @deltas = @all_events.map.with_index do |item, index|
+        delta = item.created_at - @all_events[index - 1].created_at if index > 0
         if delta == nil or delta > 10 * 60 then 0 else delta end
       end
       @working_times_until = []
-      @submissions_and_interventions.each_with_index do |submission, index|
-        @working_times_until.push((Time.at(deltas[1..index].inject(:+)).utc.strftime("%H:%M:%S") if index > 0))
+      @all_events.each_with_index do |_, index|
+        @working_times_until.push((format_time_difference(@deltas[0..index].inject(:+)) if index > 0))
       end
       render 'exercises/external_users/statistics'
     else
diff --git a/app/helpers/time_helper.rb b/app/helpers/time_helper.rb
index ed05ede5..ca93c0d8 100644
--- a/app/helpers/time_helper.rb
+++ b/app/helpers/time_helper.rb
@@ -9,4 +9,9 @@ module TimeHelper
     nil
   end
 
+  # given a delta in seconds, return a "Hours:Minutes:Seconds" representation
+  def format_time_difference(delta)
+    Time.at(delta).utc.strftime("%H:%M:%S")
+  end
+
 end
diff --git a/app/views/exercises/external_users/statistics.html.slim b/app/views/exercises/external_users/statistics.html.slim
index 7d00f3b1..5d91716a 100644
--- a/app/views/exercises/external_users/statistics.html.slim
+++ b/app/views/exercises/external_users/statistics.html.slim
@@ -39,8 +39,8 @@ h1 = "#{@exercise} (external user #{@external_user})"
             - ['.time', '.cause', '.score', '.tests', '.time_difference'].each do |title|
               th.header = t(title)
         tbody
-          - @submissions_and_interventions.each_with_index do |this, index|
-            - highlight = (index > 0 and @working_times_until[index] == @working_times_until[index - 1] and this.created_at > @submissions_and_interventions[index - 1].created_at)
+          - @all_events.each_with_index do |this, index|
+            - highlight = (index > 0 and @deltas[index] == 0 and this.created_at.to_s != @all_events[index - 1].created_at.to_s)
             tr data-id=this.id class=('highlight' if highlight)
               td.clickable = this.created_at.strftime("%F %T")
               - if this.is_a?(Submission)
@@ -49,7 +49,7 @@ h1 = "#{@exercise} (external user #{@external_user})"
                 td
                   -this.testruns.each do |run|
                     - if run.passed
-                      .unit-test-result.positive-result  title=run.output
+                      .unit-test-result.positive-result title=run.output
                     - else
                       .unit-test-result.unknown-result title=run.output
                 td = @working_times_until[index] if index > 0
@@ -57,7 +57,7 @@ h1 = "#{@exercise} (external user #{@external_user})"
                 td = this.intervention.name
                 td =
                 td =
-                td =
+                td = @working_times_until[index] if index > 0
     p = t('.addendum')
     .hidden#wtimes data-working_times=ActiveSupport::JSON.encode(@working_times_until);
     div#progress_chart.col-lg-12