Module « numpy.random »

Fonction standard_t - module numpy.random

Signature de la fonction standard_t

Description

standard_t.__doc__

        standard_t(df, size=None)

        Draw samples from a standard Student's t distribution with `df` degrees
        of freedom.

        A special case of the hyperbolic distribution.  As `df` gets
        large, the result resembles that of the standard normal
        distribution (`standard_normal`).

        .. note::
            New code should use the ``standard_t`` method of a ``default_rng()``
            instance instead; please see the :ref:`random-quick-start`.

        Parameters
        ----------
        df : float or array_like of floats
            Degrees of freedom, must be > 0.
        size : int or tuple of ints, optional
            Output shape.  If the given shape is, e.g., ``(m, n, k)``, then
            ``m * n * k`` samples are drawn.  If size is ``None`` (default),
            a single value is returned if ``df`` is a scalar.  Otherwise,
            ``np.array(df).size`` samples are drawn.

        Returns
        -------
        out : ndarray or scalar
            Drawn samples from the parameterized standard Student's t distribution.

        See Also
        --------
        Generator.standard_t: which should be used for new code.

        Notes
        -----
        The probability density function for the t distribution is

        .. math:: P(x, df) = \frac{\Gamma(\frac{df+1}{2})}{\sqrt{\pi df}
                  \Gamma(\frac{df}{2})}\Bigl( 1+\frac{x^2}{df} \Bigr)^{-(df+1)/2}

        The t test is based on an assumption that the data come from a
        Normal distribution. The t test provides a way to test whether
        the sample mean (that is the mean calculated from the data) is
        a good estimate of the true mean.

        The derivation of the t-distribution was first published in
        1908 by William Gosset while working for the Guinness Brewery
        in Dublin. Due to proprietary issues, he had to publish under
        a pseudonym, and so he used the name Student.

        References
        ----------
        .. [1] Dalgaard, Peter, "Introductory Statistics With R",
               Springer, 2002.
        .. [2] Wikipedia, "Student's t-distribution"
               https://en.wikipedia.org/wiki/Student's_t-distribution

        Examples
        --------
        From Dalgaard page 83 [1]_, suppose the daily energy intake for 11
        women in kilojoules (kJ) is:

        >>> intake = np.array([5260., 5470, 5640, 6180, 6390, 6515, 6805, 7515, \
        ...                    7515, 8230, 8770])

        Does their energy intake deviate systematically from the recommended
        value of 7725 kJ?

        We have 10 degrees of freedom, so is the sample mean within 95% of the
        recommended value?

        >>> s = np.random.standard_t(10, size=100000)
        >>> np.mean(intake)
        6753.636363636364
        >>> intake.std(ddof=1)
        1142.1232221373727

        Calculate the t statistic, setting the ddof parameter to the unbiased
        value so the divisor in the standard deviation will be degrees of
        freedom, N-1.

        >>> t = (np.mean(intake)-7725)/(intake.std(ddof=1)/np.sqrt(len(intake)))
        >>> import matplotlib.pyplot as plt
        >>> h = plt.hist(s, bins=100, density=True)

        For a one-sided t-test, how far out in the distribution does the t
        statistic appear?

        >>> np.sum(s<t) / float(len(s))
        0.0090699999999999999  #random

        So the p-value is about 0.009, which says the null hypothesis has a
        probability of about 99% of being true.