def estimate(self, significance_level=0.01):
        """
        Estimates a DAG for the data set, using the PC constraint-based
        structure learning algorithm. Independencies are identified from the
        data set using a chi-squared statistic with the acceptance threshold of
        `significance_level`. PC identifies a partially directed acyclic graph (PDAG), given
        that the tested independencies admit a faithful Bayesian network representation.
        This method returns a DAG that is a completion of this PDAG.

        Parameters
        ----------
        significance_level: float, default: 0.01
            The significance level to use for conditional independence tests in the data set.

            `significance_level` is the desired Type 1 error probability of
            falsely rejecting the null hypothesis that variables are independent,
            given that they are. The lower `significance_level`, the less likely
            we are to accept dependencies, resulting in a sparser graph.

        Returns
        -------
        model: DAG()-instance
            An estimate for the DAG for the data set (not yet parametrized).

        Reference
        ---------
        Neapolitan, Learning Bayesian Networks, Section 10.1.2, Algorithm 10.2 (page 550)
        http://www.cs.technion.ac.il/~dang/books/Learning%20Bayesian%20Networks(Neapolitan,%20Richard).pdf

        Examples
        --------
        >>> import pandas as pd
        >>> import numpy as np
        >>> from pgmpy.estimators import ConstraintBasedEstimator
        >>> data = pd.DataFrame(np.random.randint(0, 5, size=(2500, 3)), columns=list('XYZ'))
        >>> data['sum'] = data.sum(axis=1)
        >>> print(data)
              X  Y  Z  sum
        0     3  0  1    4
        1     1  4  3    8
        2     0  0  3    3
        3     0  2  3    5
        4     2  1  1    4
        ...  .. .. ..  ...
        2495  2  3  0    5
        2496  1  1  2    4
        2497  0  4  2    6
        2498  0  0  0    0
        2499  2  4  0    6

        [2500 rows x 4 columns]
        >>> c = ConstraintBasedEstimator(data)
        >>> model = c.estimate()
        >>> print(model.edges())
        [('Z', 'sum'), ('X', 'sum'), ('Y', 'sum')]
        """

        skel, separating_sets = self.estimate_skeleton(significance_level)
        pdag = self.skeleton_to_pdag(skel, separating_sets)
        model = self.pdag_to_dag(pdag)
        return model