Draw a sample of record pairs from the dataset
        (a mix of random pairs & pairs of similar records)
        and initialize active learning with this sample

        Arguments: data -- Dictionary of records, where the keys are
        record_ids and the values are dictionaries with the keys being
        field names

        sample_size         -- Size of the sample to draw
        blocked_proportion  -- Proportion of the sample that will be blocked
        original_length     -- Length of original data, should be set if `data` is
                               a sample of full data

def sample(self, data, sample_size=15000,
               blocked_proportion=0.5, original_length=None):
        '''Draw a sample of record pairs from the dataset
        (a mix of random pairs & pairs of similar records)
        and initialize active learning with this sample

        Arguments: data -- Dictionary of records, where the keys are
        record_ids and the values are dictionaries with the keys being
        field names

        sample_size         -- Size of the sample to draw
        blocked_proportion  -- Proportion of the sample that will be blocked
        original_length     -- Length of original data, should be set if `data` is
                               a sample of full data
        '''
        self._checkData(data)

        self.active_learner = self.ActiveLearner(self.data_model)
        self.active_learner.sample_combo(data, blocked_proportion,
                                         sample_size, original_length)