public function getTopKeyWordsForPeriod($startDate, $endDate, $maxResults = 30)
    {
        $keywordData = [];

        $answer = $this->performQuery($startDate, $endDate, 'ga:sessions', ['dimensions' => 'ga:keyword', 'sort' => '-ga:sessions', 'max-results' => $maxResults, 'filters' => 'ga:keyword!=(not set);ga:keyword!=(not provided)']);

        if (is_null($answer->rows)) {
            return new Collection([]);
        }

        foreach ($answer->rows as $pageRow) {
            $keywordData[] = ['keyword' => $pageRow[0], 'sessions' => $pageRow[1]];
        }

        return new Collection($keywordData);
    }