diff --git a/mod/workshop/assessment.php b/mod/workshop/assessment.php
index 528f911a0bb..cc7072b3fe7 100644
--- a/mod/workshop/assessment.php
+++ b/mod/workshop/assessment.php
@@ -106,10 +106,10 @@ if ($mform->is_cancelled()) {
         redirect($workshop->editform_url());
     }
     $rawgrade = $strategy->save_assessment($assessment, $data);
-    if (isset($data->saveandclose)) {
+    if (!is_null($rawgrade) and isset($data->saveandclose)) {
         echo $OUTPUT->header($navigation);
         echo $OUTPUT->heading(get_string('assessmentresult', 'workshop'), 2);
-        echo $OUTPUT->box('Given grade: ' . $rawgrade); // todo more detailed info using own renderer
+        echo $OUTPUT->box('Given grade: ' . sprintf("%01.2f", $rawgrade * 100) . ' %'); // todo more detailed info using own renderer
         echo $OUTPUT->continue_button($workshop->view_url());
         echo $OUTPUT->footer();
         die();  // bye-bye
diff --git a/mod/workshop/grading/accumulative/simpletest/teststrategy.php b/mod/workshop/grading/accumulative/simpletest/teststrategy.php
index d02be4bfc60..e913115b258 100644
--- a/mod/workshop/grading/accumulative/simpletest/teststrategy.php
+++ b/mod/workshop/grading/accumulative/simpletest/teststrategy.php
@@ -28,15 +28,30 @@ defined('MOODLE_INTERNAL') || die();
 // Include the code to test
 require_once($CFG->dirroot . '/mod/workshop/grading/accumulative/strategy.php');
 
+global $DB;
+Mock::generate(get_class($DB), 'mockDB');
+
 /**
  * Test subclass that makes all the protected methods we want to test public
  */
 class testable_workshop_accumulative_strategy extends workshop_accumulative_strategy {
 
+    /** allows to set dimensions manually */
+    public $dimensions = array();
+
+    /**
+     * This is where the calculation of suggested grade for submission is done
+     */
+    public function calculate_peer_grade(array $grades) {
+        return parent::calculate_peer_grade($grades);
+    }
 }
 
 class workshop_accumulative_strategy_test extends UnitTestCase {
 
+    /** real database */
+    protected $realDB;
+
     /** workshop instance emulation */
     protected $workshop;
 
@@ -47,6 +62,10 @@ class workshop_accumulative_strategy_test extends UnitTestCase {
      * Setup testing environment
      */
     public function setUp() {
+        global $DB;
+        $this->realDB   = $DB;
+        $DB             = new mockDB();
+
         $cm             = (object)array('id' => 3);
         $course         = (object)array('id' => 11);
         $workshop       = (object)array('id' => 42, 'strategy' => 'accumulative');
@@ -55,8 +74,155 @@ class workshop_accumulative_strategy_test extends UnitTestCase {
     }
 
     public function tearDown() {
+        global $DB;
+        $DB = $this->realDB;
+
         $this->workshop = null;
         $this->strategy = null;
     }
 
+    public function test_calculate_peer_grade_null_grade() {
+        // fixture set-up
+        $this->dimensions = array();
+        $grades = array();
+        // excercise SUT
+        $suggested = $this->strategy->calculate_peer_grade($grades);
+        // validate
+        $this->assertNull($suggested);
+    }
+
+    public function test_calculate_peer_grade_one_numerical() {
+        // fixture set-up
+        $this->strategy->dimensions[1003] = (object)array('grade' => 20, 'weight' => 1);
+        $grades[] = (object)array('dimensionid' => 1003, 'grade' => 5);
+        // excercise SUT
+        $suggested = $this->strategy->calculate_peer_grade($grades);
+        // validate
+        $this->assertEqual(5/20, $suggested);
+    }
+
+    public function test_calculate_peer_grade_negative_weight() {
+        // fixture set-up
+        $this->strategy->dimensions[1003] = (object)array('grade' => 20, 'weight' => -1);
+        $grades[] = (object)array('dimensionid' => 1003, 'grade' => 20);
+        $this->expectException('coding_exception');
+        // excercise SUT
+        $suggested = $this->strategy->calculate_peer_grade($grades);
+    }
+
+    public function test_calculate_peer_grade_one_numerical_weighted() {
+        // fixture set-up
+        $this->strategy->dimensions[1003] = (object)array('grade' => 20, 'weight' => 3);
+        $grades[] = (object)array('dimensionid' => 1003, 'grade' => 5);
+        // excercise SUT
+        $suggested = $this->strategy->calculate_peer_grade($grades);
+        // validate
+        $this->assertEqual(5/20, $suggested);
+    }
+
+    public function test_calculate_peer_grade_three_numericals_same_weight() {
+        // fixture set-up
+        $this->strategy->dimensions[1003] = (object)array('grade' =>20, 'weight' => 2);
+        $this->strategy->dimensions[1004] = (object)array('grade' =>100, 'weight' => 2);
+        $this->strategy->dimensions[1005] = (object)array('grade' =>10, 'weight' => 2);
+
+        $grades[] = (object)array('dimensionid' => 1003, 'grade' => 11);
+        $grades[] = (object)array('dimensionid' => 1004, 'grade' => 87);
+        $grades[] = (object)array('dimensionid' => 1005, 'grade' => 10);
+
+        // excercise SUT
+        $suggested = $this->strategy->calculate_peer_grade($grades);
+
+        // validate
+        $this->assertEqual((11/20 + 87/100 + 10/10)/3, $suggested);
+    }
+
+    public function test_calculate_peer_grade_three_numericals_different_weights() {
+        // fixture set-up
+        $this->strategy->dimensions[1003] = (object)array('grade' =>15, 'weight' => 3);
+        $this->strategy->dimensions[1004] = (object)array('grade' =>80, 'weight' => 1);
+        $this->strategy->dimensions[1005] = (object)array('grade' =>5, 'weight' => 2);
+
+        $grades[] = (object)array('dimensionid' => 1003, 'grade' => 7);
+        $grades[] = (object)array('dimensionid' => 1004, 'grade' => 66);
+        $grades[] = (object)array('dimensionid' => 1005, 'grade' => 4);
+
+        // excercise SUT
+        $suggested = $this->strategy->calculate_peer_grade($grades);
+
+        // validate
+        $this->assertEqual((7/15*3 + 66/80*1 + 4/5*2)/6, $suggested);
+    }
+
+    public function test_calculate_peer_grade_one_scale_max() {
+        global $DB;
+
+        // fixture set-up
+        $mockscale = 'E,D,C,B,A';
+        $this->strategy->dimensions[1008] = (object)array('grade' => -10, 'weight' => 1);
+        $grades[] = (object)array('dimensionid' => 1008, 'grade' => 5);
+        $DB->expectOnce('get_field', array("scales", "scale", array("id" => 10), MUST_EXIST));
+        $DB->setReturnValue('get_field', $mockscale);
+
+        // excercise SUT
+        $suggested = $this->strategy->calculate_peer_grade($grades);
+
+        // validate
+        $this->assertEqual(1, $suggested);
+    }
+
+    public function test_calculate_peer_grade_one_scale_min_with_scale_caching() {
+        global $DB;
+
+        // fixture set-up
+        $this->strategy->dimensions[1008] = (object)array('grade' => -10, 'weight' => 1);
+        $grades[] = (object)array('dimensionid' => 1008, 'grade' => 1);
+        $DB->expectNever('get_field', array("scales", "scale", array("id" => 10), MUST_EXIST)); // cached
+
+        // excercise SUT
+        $suggested = $this->strategy->calculate_peer_grade($grades);
+
+        // validate
+        $this->assertEqual(0, $suggested);
+    }
+
+    public function test_calculate_peer_grade_two_scales_weighted() {
+        global $DB;
+
+        // fixture set-up
+        $mockscale13 = 'Poor,Good,Excellent';
+        $mockscale17 = '-,*,**,***,****,*****,******';
+
+        $this->strategy->dimensions[1012] = (object)array('grade' => -13, 'weight' => 2);
+        $this->strategy->dimensions[1019] = (object)array('grade' => -17, 'weight' => 3);
+
+        $grades[] = (object)array('dimensionid' => 1012, 'grade' => 2); // "Good"
+        $grades[] = (object)array('dimensionid' => 1019, 'grade' => 5); // "****"
+
+        $DB->expectAt(0, 'get_field', array("scales", "scale", array("id" => 13), MUST_EXIST));
+        $DB->setReturnValueAt(0, 'get_field', $mockscale13);
+
+        $DB->expectAt(1, 'get_field', array("scales", "scale", array("id" => 17), MUST_EXIST));
+        $DB->setReturnValueAt(1, 'get_field', $mockscale17);
+
+        // excercise SUT
+        $suggested = $this->strategy->calculate_peer_grade($grades);
+
+        // validate
+        $this->assertEqual((1/2*2 + 4/6*3)/5, $suggested);
+    }
+
+    public function test_calculate_peer_grade_scale_exception() {
+        global $DB;
+
+        // fixture set-up
+        $mockscale13 = 'Poor,Good,Excellent';
+        $this->strategy->dimensions[1012] = (object)array('grade' => -13, 'weight' => 1);
+        $DB->expectNever('get_field', array("scales", "scale", array("id" => 13), MUST_EXIST)); // cached
+        $grades[] = (object)array('dimensionid' => 1012, 'grade' => 4); // exceeds the number of scale items
+        $this->expectException('coding_exception');
+
+        // excercise SUT
+        $suggested = $this->strategy->calculate_peer_grade($grades);
+    }
 }
diff --git a/mod/workshop/grading/accumulative/strategy.php b/mod/workshop/grading/accumulative/strategy.php
index a6587ac63d0..82d240edffc 100644
--- a/mod/workshop/grading/accumulative/strategy.php
+++ b/mod/workshop/grading/accumulative/strategy.php
@@ -195,7 +195,7 @@ class workshop_accumulative_strategy implements workshop_strategy {
     /**
      * Deletes dimensions and removes embedded media from its descriptions
      *
-     * todo we may check that there are no assessments done using these dimensions
+     * todo we may check that there are no assessments done using these dimensions and probably remove them
      *
      * @param array $masterids
      * @return void
@@ -276,7 +276,7 @@ class workshop_accumulative_strategy implements workshop_strategy {
 
         if ('assessment' === $mode and !empty($assessment)) {
             // load the previously saved assessment data
-            $grades = $DB->get_records('workshop_grades', array('assessmentid' => $assessment->id), '', 'dimensionid,*');
+            $grades = $this->reindex_grades_by_dimension($this->get_current_assessment_data($assessment));
             $current = new stdClass();
             for ($i = 0; $i < $nodimensions; $i++) {
                 $dimid = $fields->{'dimensionid__idx_'.$i};
@@ -308,7 +308,7 @@ class workshop_accumulative_strategy implements workshop_strategy {
      *
      * @param object $assessment Assessment being filled
      * @param object $data       Raw data as returned by the assessment form
-     * @return float             Percentual grade for submission as suggested by the peer
+     * @return float|null        Percentual grade for submission as suggested by the peer
      */
     public function save_assessment(stdClass $assessment, stdClass $data) {
         global $DB;
@@ -340,16 +340,128 @@ class workshop_accumulative_strategy implements workshop_strategy {
     }
 
     /**
-     * Aggregate the assessment form data and set the grade for the submission given by the peer
+     * Returns the list of current grades filled by the reviewer
      *
-     * @param stdClass $assessment Assessment record
-     * @return float               Percentual grade for submission as suggested by the peer
+     * @param object $assessment Assessment record
+     * @return array of filtered records from the table workshop_grades
      */
-    protected function update_peer_grade(stdClass $assessment) {
+    protected function get_current_assessment_data(stdClass $assessment) {
         global $DB;
 
-        $given = $DB->get_records('workshop_grades', array('assessmentid' => $assessment->id));
-        // use only grades given within the currently used strategy
-        
+        // fetch all grades accociated with this assessment
+        $grades = $DB->get_records("workshop_grades", array("assessmentid" => $assessment->id));
+
+        // filter grades given under an other strategy or assessment form
+        foreach ($grades as $grade) {
+            if (!isset($this->dimensions[$grade->dimensionid])) {
+                unset ($grades[$grade->id]);
+            }
+        }
+        return $grades;
+    }
+
+    /**
+     * Reindexes the records returned by {@link get_current_assessment_data} by dimensionid
+     *
+     * @param mixed $grades
+     * @return array
+     */
+    protected function reindex_grades_by_dimension($grades) {
+        $reindexed = array();
+        foreach ($grades as $grade) {
+            $reindexed[$grade->dimensionid] = $grade;
+        }
+        return $reindexed;
+    }
+
+    /**
+     * Aggregates the assessment form data and sets the grade for the submission given by the peer
+     *
+     * @param stdClass $assessment Assessment record
+     * @return float|null          Percentual grade for submission as suggested by the peer
+     */
+    protected function update_peer_grade(stdClass $assessment) {
+        $grades     = $this->get_current_assessment_data($assessment);
+        $suggested  = $this->calculate_peer_grade($grades);
+        if (!is_null($suggested)) {
+            // todo save into workshop_assessments
+        }
+        return $suggested;
+    }
+
+    /**
+     * Calculates the aggregated grade given by the reviewer
+     *
+     * @param array $grades Grade records as returned by {@link get_current_assessment_data}
+     * @uses $this->dimensions
+     * @return float|null   Percentual grade for submission as suggested by the peer
+     */
+    protected function calculate_peer_grade(array $grades) {
+
+        if (empty($grades)) {
+            return null;
+        }
+        $sumgrades  = 0;
+        $sumweights = 0;
+        foreach ($grades as $grade) {
+            $dimension = $this->dimensions[$grade->dimensionid];
+            if ($dimension->weight < 0) {
+                throw new coding_exception('Negative weights are not supported any more. Something is wrong with your data');
+            }
+            if ($dimension->weight == 0 or $dimension->grade == 0) {
+                // does not influence the final grade
+                continue;
+            }
+            if ($dimension->grade < 0) {
+                // this is a scale
+                $scaleid    = -$dimension->grade;
+                $sumgrades  += $this->scale_to_grade($scaleid, $grade->grade) * $dimension->weight;
+                $sumweights += $dimension->weight;
+            } else {
+                // regular grade
+                $sumgrades  += ($grade->grade / $dimension->grade) * $dimension->weight;
+                $sumweights += $dimension->weight;
+            }
+        }
+
+        if ($sumweights === 0) {
+            return 0;
+        }
+        return $sumgrades / $sumweights;
+    }
+
+    /**
+     * Convert scale grade to numerical grades
+     *
+     * In accumulative grading strategy, scales are considered as grades from 0 to M-1, where M is the number of scale items.
+     *
+     * @throws coding_exception
+     * @param string $scaleid Scale identifier
+     * @param int    $item    Selected scale item number, numbered 1, 2, 3, ... M
+     * @return float
+     */
+    protected function scale_to_grade($scaleid, $item) {
+        global $DB;
+
+        /** @var cache of numbers of scale items */
+        static $numofscaleitems = array();
+
+        if (!isset($numofscaleitems[$scaleid])) {
+            $scale = $DB->get_field("scales", "scale", array("id" => $scaleid), MUST_EXIST);
+            $items = explode(',', $scale);
+            $numofscaleitems[$scaleid] = count($items);
+            unset($scale);
+            unset($items);
+        }
+
+        if ($numofscaleitems[$scaleid] <= 1) {
+            throw new coding_exception('Invalid scale definition, no scale items found');
+        }
+
+        if ($item <= 0 or $numofscaleitems[$scaleid] < $item) {
+            throw new coding_exception('Invalid scale item number');
+        }
+
+        return ($item - 1) / ($numofscaleitems[$scaleid] - 1);
     }
 }
diff --git a/mod/workshop/grading/lib.php b/mod/workshop/grading/lib.php
index 00125cd8834..97b9da04f01 100644
--- a/mod/workshop/grading/lib.php
+++ b/mod/workshop/grading/lib.php
@@ -77,7 +77,7 @@ interface workshop_strategy {
      *
      * @param object $assessment Assessment being filled
      * @param object $data       Raw data as returned by the assessment form
-     * @return float             Percentual grade for submission as suggested by the peer
+     * @return float|float       Percentual grade for submission as suggested by the peer or null if impossible to count
      */
     public function save_assessment(stdClass $assessment, stdClass $data);
 }