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.. currentmodule:: pandas

.. ipython:: python
   :suppress:

   import numpy as np
   np.random.seed(123456)
   from pandas import *
   import pandas.util.testing as tm
   randn = np.random.randn
   np.set_printoptions(precision=4, suppress=True)
   import matplotlib.pyplot as plt
   plt.close('all')
   options.display.mpl_style='default'

Computational tools

Statistical functions

Percent Change

Both Series and DataFrame has a method pct_change to compute the percent change over a given number of periods (using fill_method to fill NA/null values).

.. ipython:: python

   ser = Series(randn(8))

   ser.pct_change()


.. ipython:: python

   df = DataFrame(randn(10, 4))

   df.pct_change(periods=3)

Covariance

The Series object has a method cov to compute covariance between series (excluding NA/null values).

.. ipython:: python

   s1 = Series(randn(1000))
   s2 = Series(randn(1000))
   s1.cov(s2)

Analogously, DataFrame has a method cov to compute pairwise covariances among the series in the DataFrame, also excluding NA/null values.

.. ipython:: python

   frame = DataFrame(randn(1000, 5), columns=['a', 'b', 'c', 'd', 'e'])
   frame.cov()

DataFrame.cov also supports an optional min_periods keyword that specifies the required minimum number of observations for each column pair in order to have a valid result.

.. ipython:: python

   frame = DataFrame(randn(20, 3), columns=['a', 'b', 'c'])
   frame.ix[:5, 'a'] = np.nan
   frame.ix[5:10, 'b'] = np.nan

   frame.cov()

   frame.cov(min_periods=12)

Correlation

Several methods for computing correlations are provided. Several kinds of correlation methods are provided:

Method name Description
pearson (default) Standard correlation coefficient
kendall Kendall Tau correlation coefficient
spearman Spearman rank correlation coefficient

All of these are currently computed using pairwise complete observations.

.. ipython:: python

   frame = DataFrame(randn(1000, 5), columns=['a', 'b', 'c', 'd', 'e'])
   frame.ix[::2] = np.nan

   # Series with Series
   frame['a'].corr(frame['b'])
   frame['a'].corr(frame['b'], method='spearman')

   # Pairwise correlation of DataFrame columns
   frame.corr()

Note that non-numeric columns will be automatically excluded from the correlation calculation.

Like cov, corr also supports the optional min_periods keyword:

.. ipython:: python

   frame = DataFrame(randn(20, 3), columns=['a', 'b', 'c'])
   frame.ix[:5, 'a'] = np.nan
   frame.ix[5:10, 'b'] = np.nan

   frame.corr()

   frame.corr(min_periods=12)

A related method corrwith is implemented on DataFrame to compute the correlation between like-labeled Series contained in different DataFrame objects.

.. ipython:: python

   index = ['a', 'b', 'c', 'd', 'e']
   columns = ['one', 'two', 'three', 'four']
   df1 = DataFrame(randn(5, 4), index=index, columns=columns)
   df2 = DataFrame(randn(4, 4), index=index[:4], columns=columns)
   df1.corrwith(df2)
   df2.corrwith(df1, axis=1)

Data ranking

The rank method produces a data ranking with ties being assigned the mean of the ranks (by default) for the group:

.. ipython:: python

   s = Series(np.random.randn(5), index=list('abcde'))
   s['d'] = s['b'] # so there's a tie
   s.rank()

rank is also a DataFrame method and can rank either the rows (axis=0) or the columns (axis=1). NaN values are excluded from the ranking.

.. ipython:: python

   df = DataFrame(np.random.randn(10, 6))
   df[4] = df[2][:5] # some ties
   df
   df.rank(1)

rank optionally takes a parameter ascending which by default is true; when false, data is reverse-ranked, with larger values assigned a smaller rank.

rank supports different tie-breaking methods, specified with the method parameter:

  • average : average rank of tied group
  • min : lowest rank in the group
  • max : highest rank in the group
  • first : ranks assigned in the order they appear in the array
.. currentmodule:: pandas


.. currentmodule:: pandas.stats.api

Moving (rolling) statistics / moments

For working with time series data, a number of functions are provided for computing common moving or rolling statistics. Among these are count, sum, mean, median, correlation, variance, covariance, standard deviation, skewness, and kurtosis. All of these methods are in the :mod:`pandas` namespace, but otherwise they can be found in :mod:`pandas.stats.moments`.

Function Description
rolling_count Number of non-null observations
rolling_sum Sum of values
rolling_mean Mean of values
rolling_median Arithmetic median of values
rolling_min Minimum
rolling_max Maximum
rolling_std Unbiased standard deviation
rolling_var Unbiased variance
rolling_skew Unbiased skewness (3rd moment)
rolling_kurt Unbiased kurtosis (4th moment)
rolling_quantile Sample quantile (value at %)
rolling_apply Generic apply
rolling_cov Unbiased covariance (binary)
rolling_corr Correlation (binary)
rolling_corr_pairwise Pairwise correlation of DataFrame columns
rolling_window Moving window function

Generally these methods all have the same interface. The binary operators (e.g. rolling_corr) take two Series or DataFrames. Otherwise, they all accept the following arguments:

These functions can be applied to ndarrays or Series objects:

.. ipython:: python

   ts = Series(randn(1000), index=date_range('1/1/2000', periods=1000))
   ts = ts.cumsum()

   ts.plot(style='k--')

   @savefig rolling_mean_ex.png width=6in
   rolling_mean(ts, 60).plot(style='k')

They can also be applied to DataFrame objects. This is really just syntactic sugar for applying the moving window operator to all of the DataFrame's columns:

.. ipython:: python
   :suppress:

   plt.close('all')


.. ipython:: python

   df = DataFrame(randn(1000, 4), index=ts.index,
                  columns=['A', 'B', 'C', 'D'])
   df = df.cumsum()

   @savefig rolling_mean_frame.png width=6in
   rolling_sum(df, 60).plot(subplots=True)

The rolling_apply function takes an extra func argument and performs generic rolling computations. The func argument should be a single function that produces a single value from an ndarray input. Suppose we wanted to compute the mean absolute deviation on a rolling basis:

.. ipython:: python

   mad = lambda x: np.fabs(x - x.mean()).mean()
   @savefig rolling_apply_ex.png width=6in
   rolling_apply(ts, 60, mad).plot(style='k')

The rolling_window function performs a generic rolling window computation on the input data. The weights used in the window are specified by the win_type keyword. The list of recognized types are:

  • boxcar
  • triang
  • blackman
  • hamming
  • bartlett
  • parzen
  • bohman
  • blackmanharris
  • nuttall
  • barthann
  • kaiser (needs beta)
  • gaussian (needs std)
  • general_gaussian (needs power, width)
  • slepian (needs width).
.. ipython:: python

   ser = Series(randn(10), index=date_range('1/1/2000', periods=10))

   rolling_window(ser, 5, 'triang')

Note that the boxcar window is equivalent to rolling_mean:

.. ipython:: python

   rolling_window(ser, 5, 'boxcar')

   rolling_mean(ser, 5)

For some windowing functions, additional parameters must be specified:

.. ipython:: python

   rolling_window(ser, 5, 'gaussian', std=0.1)

By default the labels are set to the right edge of the window, but a center keyword is available so the labels can be set at the center. This keyword is available in other rolling functions as well.

.. ipython:: python

   rolling_window(ser, 5, 'boxcar')

   rolling_window(ser, 5, 'boxcar', center=True)

   rolling_mean(ser, 5, center=True)

Binary rolling moments

rolling_cov and rolling_corr can compute moving window statistics about two Series or any combination of DataFrame/Series or DataFrame/DataFrame. Here is the behavior in each case:

  • two Series: compute the statistic for the pairing
  • DataFrame/Series: compute the statistics for each column of the DataFrame with the passed Series, thus returning a DataFrame
  • DataFrame/DataFrame: compute statistic for matching column names, returning a DataFrame

For example:

.. ipython:: python

   df2 = df[:20]
   rolling_corr(df2, df2['B'], window=5)

Computing rolling pairwise correlations

In financial data analysis and other fields it's common to compute correlation matrices for a collection of time series. More difficult is to compute a moving-window correlation matrix. This can be done using the rolling_corr_pairwise function, which yields a Panel whose items are the dates in question:

.. ipython:: python

   correls = rolling_corr_pairwise(df, 50)
   correls[df.index[-50]]

You can efficiently retrieve the time series of correlations between two columns using ix indexing:

.. ipython:: python
   :suppress:

   plt.close('all')


.. ipython:: python

   @savefig rolling_corr_pairwise_ex.png width=6in
   correls.ix[:, 'A', 'C'].plot()

Expanding window moment functions

A common alternative to rolling statistics is to use an expanding window, which yields the value of the statistic with all the data available up to that point in time. As these calculations are a special case of rolling statistics, they are implemented in pandas such that the following two calls are equivalent:

.. ipython:: python

   rolling_mean(df, window=len(df), min_periods=1)[:5]

   expanding_mean(df)[:5]

Like the rolling_ functions, the following methods are included in the pandas namespace or can be located in pandas.stats.moments.

Function Description
expanding_count Number of non-null observations
expanding_sum Sum of values
expanding_mean Mean of values
expanding_median Arithmetic median of values
expanding_min Minimum
expanding_max Maximum
expanding_std Unbiased standard deviation
expanding_var Unbiased variance
expanding_skew Unbiased skewness (3rd moment)
expanding_kurt Unbiased kurtosis (4th moment)
expanding_quantile Sample quantile (value at %)
expanding_apply Generic apply
expanding_cov Unbiased covariance (binary)
expanding_corr Correlation (binary)
expanding_corr_pairwise Pairwise correlation of DataFrame columns

Aside from not having a window parameter, these functions have the same interfaces as their rolling_ counterpart. Like above, the parameters they all accept are:

  • min_periods: threshold of non-null data points to require. Defaults to minimum needed to compute statistic. No NaNs will be output once min_periods non-null data points have been seen.
  • freq: optionally specify a :ref:`frequency string <timeseries.alias>` or :ref:`DateOffset <timeseries.offsets>` to pre-conform the data to. Note that prior to pandas v0.8.0, a keyword argument time_rule was used instead of freq that referred to the legacy time rule constants

Note

The output of the rolling_ and expanding_ functions do not return a NaN if there are at least min_periods non-null values in the current window. This differs from cumsum, cumprod, cummax, and cummin, which return NaN in the output wherever a NaN is encountered in the input.

An expanding window statistic will be more stable (and less responsive) than its rolling window counterpart as the increasing window size decreases the relative impact of an individual data point. As an example, here is the expanding_mean output for the previous time series dataset:

.. ipython:: python
   :suppress:

   plt.close('all')


.. ipython:: python

   ts.plot(style='k--')

   @savefig expanding_mean_frame.png width=6in
   expanding_mean(ts).plot(style='k')

Exponentially weighted moment functions

A related set of functions are exponentially weighted versions of many of the above statistics. A number of EW (exponentially weighted) functions are provided using the blending method. For example, where y_t is the result and x_t the input, we compute an exponentially weighted moving average as

y_t = \alpha y_{t-1} + (1 - \alpha) x_t

One must have 0 < \alpha \leq 1, but rather than pass \alpha directly, it's easier to think about either the span or center of mass (com) of an EW moment:

\alpha =
 \begin{cases}
     \frac{2}{s + 1}, s = \text{span}\\
     \frac{1}{c + 1}, c = \text{center of mass}
 \end{cases}

You can pass one or the other to these functions but not both. Span corresponds to what is commonly called a "20-day EW moving average" for example. Center of mass has a more physical interpretation. For example, span = 20 corresponds to com = 9.5. Here is the list of functions available:

Function Description
ewma EW moving average
ewmvar EW moving variance
ewmstd EW moving standard deviation
ewmcorr EW moving correlation
ewmcov EW moving covariance

Here are an example for a univariate time series:

.. ipython:: python

   plt.close('all')
   ts.plot(style='k--')

   @savefig ewma_ex.png width=6in
   ewma(ts, span=20).plot(style='k')

Note

The EW functions perform a standard adjustment to the initial observations whereby if there are fewer observations than called for in the span, those observations are reweighted accordingly.