.. -*- coding: utf-8 -*-

.. linkall

.. _prep-quickstart-statistics-and-distributions:

Sage Quickstart for Statistics
==============================

This `Sage <https://www.sagemath.org>`_ quickstart tutorial was developed
for the MAA PREP Workshop "Sage: Using Open\-Source Mathematics Software
with Undergraduates" (funding provided by NSF DUE 0817071).  It is
licensed under the Creative Commons Attribution\-ShareAlike 3.0 license
(`CC BY\-SA <https://creativecommons.org/licenses/by-sa/3.0/>`_).


Basic Descriptive Statistics
----------------------------

Basic statistical functions are best to get from `numpy <https://numpy.org/doc/stable/reference/routines.statistics.html>`_
and `scipy.stats <https://docs.scipy.org/doc/scipy/reference/stats.html>`_.

NumPy provides, for example, functions to compute the arithmetic mean and
the standard deviation::

    sage: import numpy as np
    sage: np.mean([1, 2, 3, 5])
    2.75

    sage: np.std([1, 2, 2, 4, 5, 6, 8])  # rel to 1e-13
    2.32992949004287

SciPy offers many more functions, for example a function to compute the
harmonic mean::

    sage: from scipy.stats import hmean
    sage: hmean([1, 2, 2, 4, 5, 6, 8])  # rel to 1e-13
    2.5531914893617023

We do not recommend to use Python's built in ``statistics`` module with Sage.
It has a known incompatibility with number types defined in Sage, see :trac:`28234`.


Distributions
-------------

Let's generate a random sample from a given type of continuous
distribution using native Python random generators.

- We use the simplest method of generating random elements from a log
  normal distribution with (normal) mean 2 and :math:`\sigma=3`.

- Notice that there is really no way around making some kind of loop.

::

    sage: my_data = [lognormvariate(2, 3) for i in range(10)]
    sage: my_data # random
    [13.189347821530054, 151.28229284782799, 0.071974845847761343, 202.62181449742425, 1.9677158880100207, 71.959830176932542, 21.054742855786007, 3.9235315623286406, 4129.9880239483346, 16.41063858663054]

We can check whether the mean of the log of the data is close to 2.

::

    sage: np.mean([log(item) for item in my_data]) # random
    3.0769518857697618

Here is an example using the `Gnu scientific library
<http://www.gnu.org/software/gsl/>`_ under the hood.

- Let ``dist`` be the variable assigned to a continuous Gaussian/normal
  distribution with standard deviation of 3.

- Then we use the ``.get_random_element()`` method ten times, adding 2
  each time so that the mean is equal to 2.

::

    sage: dist=RealDistribution('gaussian',3)
    sage: my_data=[dist.get_random_element()+2 for _ in range(10)]
    sage: my_data # random
    [3.18196848067, -2.70878671264, 0.445500746768, 0.579932075555, -1.32546445128, 0.985799587162, 4.96649083229, -1.78785287243, -3.05866866979, 5.90786474822]

For now, it's a little annoying to get histograms of such things
directly. Here, we get a larger sampling of this distribution and
plot a histogram with 10 bins.

::

    sage: my_data2 = [dist.get_random_element()+2 for _ in range(1000)]
    sage: T = stats.TimeSeries(my_data)
    sage: T.plot_histogram(normalize=False,bins=10)
    Graphics object consisting of 10 graphics primitives

To access discrete distributions, we again use `Scipy <http://www.scipy.org>`_.

- We have to ``import`` the module ``scipy.stats``.

- We use ``binom_dist`` to denote the binomial distribution with 20 trials
  and 5% expected failure rate.

- The ``.pmf(x)`` method gives the probability of :math:`x` failures,
  which we then plot in a bar chart for :math:`x` from 0 to 20.
  (Don't forget that ``range(21)`` means all integers from *zero to twenty*.)

::

    sage: import scipy.stats
    sage: binom_dist = scipy.stats.binom(20,.05)
    sage: bar_chart([binom_dist.pmf(x) for x in range(21)])
    Graphics object consisting of 1 graphics primitive

The ``bar_chart`` function performs some of the duties of histograms.

Scipy's statistics can do other things too.  Here, we find the median
(as the fiftieth percentile) of an earlier data set.

::

    sage: scipy.stats.scoreatpercentile(my_data, 50) # random
    0.51271641116183286

The key thing to remember here is to look at the documentation!


Using R from within Sage
------------------------

The `R project <http://www.r-project.org>`_ is a leading software package
for statistical analysis with widespread use in industry and academics.

There are several ways to access R; they are based on the ``rpy2`` package.

- One of the easiest is to just put ``r()`` around things you want to
  make into statistical objects, and then ...

- Use R commands via ``r.method()`` to pass them on to Sage for further
  processing.

The following example of the Kruskal\-Wallis test comes directly from
the examples in ``r.kruskal_test?`` in the notebook.

::

    sage: x=r([2.9, 3.0, 2.5, 2.6, 3.2])  # normal subjects                       # optional - rpy2
    sage: y=r([3.8, 2.7, 4.0, 2.4])       # with obstructive airway disease       # optional - rpy2
    sage: z=r([2.8, 3.4, 3.7, 2.2, 2.0])  # with asbestosis                       # optional - rpy2
    sage: a = r([x,y,z])                  # make a long R vector of all the data  # optional - rpy2
    sage: b = r.factor(5*[1]+4*[2]+5*[3]) # create something for R to tell        # optional - rpy2
    ....:                                 #   which subjects are which
    sage: a; b                            # show them                             # optional - rpy2
     [1] 2.9 3.0 2.5 2.6 3.2 3.8 2.7 4.0 2.4 2.8 3.4 3.7 2.2 2.0
     [1] 1 1 1 1 1 2 2 2 2 3 3 3 3 3
    Levels: 1 2 3

.. skip

::

    sage: r.kruskal_test(a,b)             # do the KW test!                       # optional - rpy2
        Kruskal-Wallis rank sum test

    data:  sage17 and sage33
    Kruskal-Wallis chi-squared = 0.7714, df = 2, p-value = 0.68

Looks like we can't reject the null hypothesis here.

The best way to use R seriously is to simply ask each individual cell to
evaluate completely in R, using a so\-called "percent directive".  Here
is a sample linear regression from John Verzani's `simpleR
<http://cran.r-project.org/doc/contrib/Verzani-SimpleR.pdf>`_ text.
Notice that R also uses the ``#`` symbol to indicate comments.

.. skip

::

    sage: %r                                                                      # optional - rpy2
    ....: x = c(18,23,25,35,65,54,34,56,72,19,23,42,18,39,37) # ages of individuals
    ....: y = c(202,186,187,180,156,169,174,172,153,199,193,174,198,183,178) # maximum heart rate of each one
    ....: png() # turn on plotting
    ....: plot(x,y) # make a plot
    ....: lm(y ~ x) # do the linear regression
    ....: abline(lm(y ~ x)) # plot the regression line
    ....: dev.off()     # turn off the device so it plots
    Call:
    lm(formula = y ~ x)

    Coefficients:
    (Intercept)            x
       210.0485      -0.7977

    null device
              1

.. image:: ../media/Rplot001.png
    :align: center

To get a whole worksheet to evaluate in R (and be able to ignore the
``%``), you could also drop down the ``r`` option in the menu close to
the top which currently has ``sage`` in it.
