diff --git a/mod/workshop/eval/best/lang/en/workshopeval_best.php b/mod/workshop/eval/best/lang/en/workshopeval_best.php
index 7e0b63ca139..6af9f37dbaf 100644
--- a/mod/workshop/eval/best/lang/en/workshopeval_best.php
+++ b/mod/workshop/eval/best/lang/en/workshopeval_best.php
@@ -33,4 +33,3 @@ $string['comparisonlevel7'] = 'lax';
 $string['comparisonlevel9'] = 'very lax';
 $string['configcomparison'] = 'Default value of the factor that influence the grading evaluation.';
 $string['pluginname'] = 'Comparison with the best assessment';
-$string['settings'] = 'Grading evaluation settings';
diff --git a/mod/workshop/eval/best/lib.php b/mod/workshop/eval/best/lib.php
index 0857396756c..546b0e21cc9 100644
--- a/mod/workshop/eval/best/lib.php
+++ b/mod/workshop/eval/best/lib.php
@@ -33,7 +33,7 @@ require_once($CFG->libdir . '/gradelib.php');
 /**
  * Defines the computation login of the grading evaluation subplugin
  */
-class workshop_best_evaluation implements workshop_evaluation {
+class workshop_best_evaluation extends workshop_evaluation {
 
     /** @var workshop the parent workshop instance */
     protected $workshop;
@@ -67,7 +67,7 @@ class workshop_best_evaluation implements workshop_evaluation {
     public function update_grading_grades(stdclass $settings, $restrict=null) {
         global $DB;
 
-        // remember the recently used settings for this workshop
+        // Remember the recently used settings for this workshop.
         if (empty($this->settings)) {
             $record = new stdclass();
             $record->workshopid = $this->workshop->id;
@@ -78,7 +78,7 @@ class workshop_best_evaluation implements workshop_evaluation {
                     array('workshopid' => $this->workshop->id));
         }
 
-        // get the grading strategy instance
+        // Get the grading strategy instance.
         $grader = $this->workshop->grading_strategy_instance();
 
         // get the information about the assessment dimensions
@@ -109,14 +109,11 @@ class workshop_best_evaluation implements workshop_evaluation {
     }
 
     /**
-     * TODO: short description.
+     * Returns an instance of the form to provide evaluation settings.
      *
-     * @return TODO
+     * @return workshop_best_evaluation_settings_form
      */
     public function get_settings_form(moodle_url $actionurl=null) {
-        global $CFG;    // needed because the included files use it
-        global $DB;
-        require_once(dirname(__FILE__) . '/settings_form.php');
 
         $customdata['workshop'] = $this->workshop;
         $customdata['current'] = $this->settings;
@@ -403,3 +400,30 @@ class workshop_best_evaluation implements workshop_evaluation {
         }
     }
 }
+
+
+/**
+ * Represents the settings form for this plugin.
+ */
+class workshop_best_evaluation_settings_form extends workshop_evaluation_settings_form {
+
+    /**
+     * Defines specific fields for this evaluation method.
+     */
+    protected function definition_sub() {
+        $mform = $this->_form;
+
+        $plugindefaults = get_config('workshopeval_best');
+        $current = $this->_customdata['current'];
+
+        $options = array();
+        for ($i = 9; $i >= 1; $i = $i-2) {
+            $options[$i] = get_string('comparisonlevel' . $i, 'workshopeval_best');
+        }
+        $mform->addElement('select', 'comparison', get_string('comparison', 'workshopeval_best'), $options);
+        $mform->addHelpButton('comparison', 'comparison', 'workshopeval_best');
+        $mform->setDefault('comparison', $plugindefaults->comparison);
+
+        $this->set_data($current);
+    }
+}
diff --git a/mod/workshop/eval/best/settings_form.php b/mod/workshop/eval/best/settings_form.php
deleted file mode 100644
index 9352ef1c7db..00000000000
--- a/mod/workshop/eval/best/settings_form.php
+++ /dev/null
@@ -1,59 +0,0 @@
-<?php
-
-// This file is part of Moodle - http://moodle.org/
-//
-// Moodle is free software: you can redistribute it and/or modify
-// it under the terms of the GNU General Public License as published by
-// the Free Software Foundation, either version 3 of the License, or
-// (at your option) any later version.
-//
-// Moodle is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-// GNU General Public License for more details.
-//
-// You should have received a copy of the GNU General Public License
-// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
-
-/**
- * Best evaluation settings form
- *
- * @package    workshopeval
- * @subpackage best
- * @copyright  2009 David Mudrak <david.mudrak@gmail.com>
- * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
- */
-
-defined('MOODLE_INTERNAL') || die();
-
-require_once($CFG->dirroot . '/lib/formslib.php');
-
-class workshop_best_evaluation_settings_form extends moodleform {
-
-    function definition() {
-        $mform = $this->_form;
-
-        $plugindefaults = get_config('workshopeval_best');
-        $current        = $this->_customdata['current'];
-        $workshop       = $this->_customdata['workshop'];
-
-        $mform->addElement('header', 'general', get_string('settings', 'workshopeval_best'));
-
-        $label = get_string('evaluationmethod', 'workshop');
-        $mform->addElement('static', 'methodname', $label, get_string('pluginname', 'workshopeval_best'));
-        $mform->addHelpButton('methodname', 'evaluationmethod', 'workshop');
-
-        $options = array();
-        for ($i = 9; $i >= 1; $i = $i-2) {
-            $options[$i] = get_string('comparisonlevel' . $i, 'workshopeval_best');
-        }
-        $label = get_string('comparison', 'workshopeval_best');
-        $mform->addElement('select', 'comparison', $label, $options);
-        $mform->addHelpButton('comparison', 'comparison', 'workshopeval_best');
-        $mform->setDefault('comparison', $plugindefaults->comparison);
-
-        $mform->addElement('submit', 'submit', get_string('aggregategrades', 'workshop'));
-
-        $this->set_data($current);
-    }
-}
diff --git a/mod/workshop/eval/lib.php b/mod/workshop/eval/lib.php
index bfec05f945d..82eeafbcedb 100644
--- a/mod/workshop/eval/lib.php
+++ b/mod/workshop/eval/lib.php
@@ -26,12 +26,37 @@
 
 defined('MOODLE_INTERNAL') || die();
 
+require_once($CFG->dirroot . '/lib/formslib.php');
+
 /**
- * Defines all methods that grading evaluation subplugins has to implement
- *
- * @todo the final interface is not decided yet as we have only one implementation so far
+ * Base class for all grading evaluation subplugins.
  */
-interface workshop_evaluation {
+abstract class workshop_evaluation {
+
+    /**
+     * Calculates grades for assessment and updates 'gradinggrade' fields in 'workshop_assessments' table
+     *
+     * @param stdClass $settings settings for this round of evaluation
+     * @param null|int|array $restrict if null, update all reviewers, otherwise update just grades for the given reviewers(s)
+     */
+    abstract public function update_grading_grades(stdClass $settings, $restrict=null);
+
+    /**
+     * Returns an instance of the form to provide evaluation settings.
+      *
+     * This is called by view.php (to display) and aggregate.php (to process and dispatch).
+     * It returns the basic form with just the submit button by default. Evaluators may
+     * extend or overwrite the default form to include some custom settings.
+     *
+     * @return workshop_evaluation_settings_form
+     */
+    public function get_settings_form(moodle_url $actionurl=null) {
+
+        $customdata = array('workshop' => $this->workshop);
+        $attributes = array('class' => 'evalsettingsform');
+
+        return new workshop_evaluation_settings_form($actionurl, $customdata, 'post', '', $attributes);
+    }
 
     /**
      * Delete all data related to a given workshop module instance
@@ -41,5 +66,35 @@ interface workshop_evaluation {
      * @param int $workshopid id of the workshop module instance being deleted
      * @return void
      */
-    public static function delete_instance($workshopid);
+    public static function delete_instance($workshopid) {
+
+    }
+}
+
+
+/**
+ * Base form to hold eventual evaluation settings.
+ */
+class workshop_evaluation_settings_form extends moodleform {
+
+    /**
+     * Defines the common form fields.
+     */
+    public function definition() {
+        $mform = $this->_form;
+
+        $workshop = $this->_customdata['workshop'];
+
+        $mform->addElement('header', 'general', get_string('evaluationsettings', 'mod_workshop'));
+
+        $this->definition_sub();
+
+        $mform->addElement('submit', 'submit', get_string('aggregategrades', 'workshop'));
+    }
+
+    /**
+     * Defines the subplugin specific fields.
+     */
+    protected function definition_sub() {
+    }
 }
diff --git a/mod/workshop/lang/en/workshop.php b/mod/workshop/lang/en/workshop.php
index 6efe8498e6d..f2fbd0dead1 100644
--- a/mod/workshop/lang/en/workshop.php
+++ b/mod/workshop/lang/en/workshop.php
@@ -100,7 +100,8 @@ $string['err_removegrademappings'] = 'Unable to remove the unused grade mappings
 $string['evaluategradeswait'] = 'Please wait until the assessments are evaluated and the grades are calculated';
 $string['evaluation'] = 'Grading evaluation';
 $string['evaluationmethod'] = 'Grading evaluation method';
-$string['evaluationmethod_help'] = 'The grading evaluation method determines how the grade for assessment is calculated. There is currently just one option - comparison with the best assessment.';
+$string['evaluationmethod_help'] = 'The grading evaluation method determines how the grade for assessment is calculated. You can let it re-calculate grades repeatedly with different settings unless you are happy with the result.';
+$string['evaluationsettings'] = 'Grading evaluation settings';
 $string['example'] = 'Example submission';
 $string['exampleadd'] = 'Add example submission';
 $string['exampleassess'] = 'Assess example submission';
diff --git a/mod/workshop/lib.php b/mod/workshop/lib.php
index b66bd4fcf7b..7250fe36676 100644
--- a/mod/workshop/lib.php
+++ b/mod/workshop/lib.php
@@ -136,7 +136,6 @@ function workshop_update_instance(stdclass $workshop) {
     $workshop->useselfassessment     = (int)!empty($workshop->useselfassessment);
     $workshop->latesubmissions       = (int)!empty($workshop->latesubmissions);
     $workshop->phaseswitchassessment = (int)!empty($workshop->phaseswitchassessment);
-    $workshop->evaluation            = 'best';
 
     // todo - if the grading strategy is being changed, we must replace all aggregated peer grades with nulls
     // todo - if maximum grades are being changed, we should probably recalculate or invalidate them
diff --git a/mod/workshop/locallib.php b/mod/workshop/locallib.php
index 340e771d179..20c579d98e8 100644
--- a/mod/workshop/locallib.php
+++ b/mod/workshop/locallib.php
@@ -185,7 +185,6 @@ class workshop {
         } else {
             $this->context = $context;
         }
-        $this->evaluation   = 'best';   // todo make this configurable although we have no alternatives yet
     }
 
     ////////////////////////////////////////////////////////////////////////////////
@@ -283,6 +282,19 @@ class workshop {
         return $forms;
     }
 
+    /**
+     * Returns the list of available grading evaluation methods
+     *
+     * @return array of (string)name => (string)localized title
+     */
+    public static function available_evaluators_list() {
+        $evals = array();
+        foreach (get_plugin_list_with_file('workshopeval', 'lib.php', false) as $eval => $evalpath) {
+            $evals[$eval] = get_string('pluginname', 'workshopeval_' . $eval);
+        }
+        return $evals;
+    }
+
     /**
      * Return an array of possible values of assessment dimension weight
      *
@@ -1229,6 +1241,28 @@ class workshop {
         return $this->strategyinstance;
     }
 
+    /**
+     * Sets the current evaluation method to the given plugin.
+     *
+     * @param string $method the name of the workshopeval subplugin
+     * @return bool true if successfully set
+     * @throws coding_exception if attempting to set a non-installed evaluation method
+     */
+    public function set_grading_evaluation_method($method) {
+        global $DB;
+
+        $evaluationlib = dirname(__FILE__) . '/eval/' . $method . '/lib.php';
+
+        if (is_readable($evaluationlib)) {
+            $this->evaluationinstance = null;
+            $this->evaluation = $method;
+            $DB->set_field('workshop', 'evaluation', $method, array('id' => $this->id));
+            return true;
+        }
+
+        throw new coding_exception('Attempt to set a non-existing evaluation method.');
+    }
+
     /**
      * Returns instance of grading evaluation class
      *
@@ -1238,16 +1272,27 @@ class workshop {
         global $CFG;    // because we require other libs here
 
         if (is_null($this->evaluationinstance)) {
+            if (empty($this->evaluation)) {
+                $this->evaluation = 'best';
+            }
             $evaluationlib = dirname(__FILE__) . '/eval/' . $this->evaluation . '/lib.php';
             if (is_readable($evaluationlib)) {
                 require_once($evaluationlib);
             } else {
-                throw new coding_exception('the grading evaluation subplugin must contain library ' . $evaluationlib);
+                // Fall back in case the subplugin is not available.
+                $this->evaluation = 'best';
+                $evaluationlib = dirname(__FILE__) . '/eval/' . $this->evaluation . '/lib.php';
+                if (is_readable($evaluationlib)) {
+                    require_once($evaluationlib);
+                } else {
+                    // Fall back in case the subplugin is not available any more.
+                    throw new coding_exception('Missing default grading evaluation library ' . $evaluationlib);
+                }
             }
             $classname = 'workshop_' . $this->evaluation . '_evaluation';
             $this->evaluationinstance = new $classname($this);
-            if (!in_array('workshop_evaluation', class_implements($this->evaluationinstance))) {
-                throw new coding_exception($classname . ' does not implement workshop_evaluation interface');
+            if (!in_array('workshop_evaluation', class_parents($this->evaluationinstance))) {
+                throw new coding_exception($classname . ' does not extend workshop_evaluation class');
             }
         }
         return $this->evaluationinstance;
diff --git a/mod/workshop/styles.css b/mod/workshop/styles.css
index 6f9c01409ce..88e13362c2b 100644
--- a/mod/workshop/styles.css
+++ b/mod/workshop/styles.css
@@ -520,3 +520,8 @@
 .path-mod-workshop div.buttonwithhelp div {
     display: inline;
 }
+
+.path-mod-workshop #evaluationmethodchooser {
+    margin: 2em auto;
+    text-align: center;
+}
diff --git a/mod/workshop/view.php b/mod/workshop/view.php
index e5817dd263f..43db74407a7 100644
--- a/mod/workshop/view.php
+++ b/mod/workshop/view.php
@@ -38,6 +38,7 @@ $page       = optional_param('page', 0, PARAM_INT);
 $perpage    = optional_param('perpage', null, PARAM_INT);
 $sortby     = optional_param('sortby', 'lastname', PARAM_ALPHA);
 $sorthow    = optional_param('sorthow', 'ASC', PARAM_ALPHA);
+$eval       = optional_param('eval', null, PARAM_PLUGIN);
 
 if ($id) {
     $cm         = get_coursemodule_from_id('workshop', $id, 0, false, MUST_EXIST);
@@ -73,6 +74,13 @@ if ($perpage and $perpage > 0 and $perpage <= 1000) {
     redirect($PAGE->url);
 }
 
+if ($eval) {
+    require_sesskey();
+    require_capability('mod/workshop:overridegrades', $workshop->context);
+    $workshop->set_grading_evaluation_method($eval);
+    redirect($PAGE->url);
+}
+
 $output = $PAGE->get_renderer('mod_workshop');
 $userplan = new workshop_user_plan($workshop, $USER->id);
 
@@ -395,6 +403,13 @@ case workshop::PHASE_EVALUATION:
             $showreviewernames  = has_capability('mod/workshop:viewreviewernames', $workshop->context);
 
             if (has_capability('mod/workshop:overridegrades', $PAGE->context)) {
+                // Print a drop-down selector to change the current evaluation method.
+                $selector = new single_select($PAGE->url, 'eval', workshop::available_evaluators_list(),
+                    $workshop->evaluation, false, 'evaluationmethodchooser');
+                $selector->set_label(get_string('evaluationmethod', 'mod_workshop'));
+                $selector->set_help_icon('evaluationmethod', 'mod_workshop');
+                $selector->method = 'post';
+                echo $output->render($selector);
                 // load the grading evaluator
                 $evaluator = $workshop->grading_evaluation_instance();
                 $form = $evaluator->get_settings_form(new moodle_url($workshop->aggregate_url(),