def format_output(self, child_key):
        """ Formats the data.

        self.stats1 contains a list of one or two pstat.Stats() instances, with
        the first being the current run and the second, the saved run, if it
        exists.  Each Stats instance is a dictionary mapping a function to
        5 data points - cumulative calls, number of calls, total time,
        cumulative time, and callers.

        format_output() converts the number of calls, total time, and
        cumulative time to a string format for the child_key parameter.
        """
        data = [x.stats.get(child_key, [0, 0, 0, 0, {}]) for x in self.stats1]
        return (map(self.color_string, islice(zip(*data), 1, 4)))