'Returns the summary value for the data in this column.
It returns the summary value if a valid summation method is
specified for it. Otherwise returns ``None``.'

def get_summation(self):

    if (self.summation not in self.summation_methods):
        return None
    summation_function = self.summation_methods[self.summation]
    data = [self.get_raw_data(datum) for datum in self.table.data]
    data = [raw_data for raw_data in data if (raw_data is not None)]
    if len(data):
        try:
            summation = summation_function(data)
            for filter_func in self.filters:
                summation = filter_func(summation)
            return summation
        except TypeError:
            pass
    return None
