def click_rate_as_publisher(start_date = nil, end_date = nil)
      impressions_count = impressions_count_as_publisher(start_date, end_date)
      return 0 if impressions_count.zero?
      clicks_count = clicks_count_as_publisher(start_date, end_date)
      (clicks_count / impressions_count.to_f) * 100