// sum risk scores for each birth year
        // $birth_years is array of birth_year=>sum_of_score
    // $years_count is birth_year=>how_many_times_it_occurs
    foreach($visitors as $visitor) {
      $birth_years[$visitor['birth_year']] += $visitor['risk_score'];
      $years_count[$visitor['birth_year']]++;
    }

    // divide sums by times that year occurs to get avg
    array_walk($birth_years, function($val, $key) use($years_count) {
      error_log(print_r($key, true), 0);
      error_log(print_r($val, true), 0);
      error_log(print_r($years_count[$key], true), 0);
      $val = $val / $years_count[$key];
    });