bolt->birth_year) return true; if (!$user->bolt->sex) return true; return false; } function birth_year_select($user) { $this_year = date("Y"); $x = "\n"; for ($i=$this_year-100; $i<$this_year; $i++) { $s = ($i == $user->bolt->birth_year)?"selected":""; $x .= "$i\n"; } $s = (!$user->bolt->birth_year)?"selected":""; $x .= "Unspecified\n"; $x .= "\n"; return $x; } function sex_select($user) { $x = "\n"; $s = ($user->bolt->sex == 0)?"selected":""; $x .= "Unspecified\n"; $s = ($user->bolt->sex == 1)?"selected":""; $x .= "Male\n"; $s = ($user->bolt->sex == 2)?"selected":""; $x .= "Female\n"; $x .= "\n"; return $x; } function request_info($user, $course) { page_head("About you"); echo " You may optionally tell us some facts about yourself. This information will help us improve this course, and will be kept private.

id> "; start_table(); row2("Birth year", birth_year_select($user)); row2("Sex", sex_select($user)); row2("", ""); end_table(); echo "

\n"; page_tail(); } ////// stuff related to snapshots // a "snapshot" is a condensed representation of the results // for a particular select/xset pair. // Namely, it's an array whose elements contain // bolt_user: the user // xset_result: the user's first completion of the xset // select_finished: the user's last completion of the select before this function write_snapshot($course_id, $select_name, $xset_name, $start) { $xrs = BoltXsetResult::enum( "course_id=$course_id and name='$xset_name' and create_time >= $start" ); $sfs = BoltSelectFinished::enum( "course_id=$course_id and name='$select_name' and end_time >= $start" ); // make an array, keyed by user ID, of earliest xset result // $a = array(); foreach ($xrs as $xr) { $uid = $xr->user_id; if (!array_key_exists($uid, $a) || $xr->create_time < $a[$uid]->xr->create_time) { $x = null; $x->xr = $xr; $a[$uid] = $x; } } // now scan select finishes, and for each user find last one before xset // foreach ($sfs as $sf) { $uid = $sf->user_id; if (!array_key_exists($uid, $a)) continue; $x = $a[$uid]; $xr = $x->xr; if ($sf->end_time > $xr->create_time) continue; $s = $x->sf; if (!is_set($x->sf) || $sf->create_time > $s.create_time) { $x->sf = $sf; $a[$uid] = $x; } } $filename = "compare_snapshot_$course_id_$select_name_$xset_name.json"; $f = fopen($filename, "w"); fwrite($f, json_encode($a)); fclose($f); } function read_snapshot($course_id, $select_name, $xset_name) { $filename = "compare_snapshot_$course_id_$select_name_$xset_name.json"; $f = fopen($filename, "r"); $x = fread($f, filesize($filename)); fclose($f); return json_decode($x); } ////// Statistics // compute the mean and stdev of an array // function mean_stdev($array, &$mean, &$stdev) { $n = 0; $m = 0; $m2 = 0; foreach ($array as $x) { $n++; $delta = $x - $m; $m += $delta/$n; $m2 += $delta*($x-$m); } $mean = $m; $stdev = sqrt($m2/($n-1)); } // approximate the 90% confidence interval for the mean of an array // function conf_int_90($array, &$lo, &$hi) { $n = count($array); mean_stdev($array, $mean, $stdev); // I'm too lazy to compute the t distribution $t_90 = 1.7; $d = $t_90 * $stdev / sqrt($n); $lo = $mean - $d; $hi = $mean + $d; } function test_stats() { $a = array(1,1,1,1,0,1,1,1,3, 1, 1, 1, 1); mean_stdev($a, $mean, $stdev); echo "mean: $mean stdev: $stdev\n"; conf_int_90($a, $lo, $hi); echo "lo $lo hi $hi\n"; } ?>