def add_sales_variance(bigTable):
    """ Adds a new column reporting the variance
    in unit_sales for each (item, store) tuple
    """
    df = bigTable.groupby(['store_nbr', 'item_nbr'])['unit_sales']\
        .var().reset_index()
    return bigTable.merge(df.rename(columns={'unit_sales':
                                             'item_store_sales_variance'
                                             }), on=['store_nbr', 'item_nbr'])