def calc_gross_profit_margin(revenue_time_series, cogs_time_series):  # Profit and Cost of Goods Sold - i.e. cost of materials and director labour costs
    """
        Gross Profit Margins Formula
        Notes
        ------------
            Profit Margins =  Total revenue - Cost of goods sold (COGS) / revenue
            = Profit / Revenue = Cost of Materials and Direct Labour Costs
    """
    gross_profit_margin = (revenue_time_series - cogs_time_series) / revenue_time_series
    return gross_profit_margin