def remove_companies_without_valid_isin(data, isin_column):
    """ Removes any row without a ISIN number. """

    old_length = len(data[isin_column])

    data.dropna(subset=[isin_column], inplace=True)

    new_length = len(data[isin_column])
    percentage = 1.0 - new_length / old_length

    print(
        f"Removed {old_length - new_length} records without valid ISIN (reduction of {percentage:.2%})"
    )

    return data