# Astrostatistics Tools (astropy.stats)#

## Introduction#

The astropy.stats package holds statistical functions or algorithms used in astronomy. While the scipy.stats and statsmodels packages contains a wide range of statistical tools, they are general-purpose packages and are missing some tools that are particularly useful or specific to astronomy. This package is intended to provide such functionality, but not to replace scipy.stats if its implementation satisfies astronomers’ needs.

## Getting Started#

A number of different tools are contained in the stats package, and they can be accessed by importing them:

>>> from astropy import stats


A full list of the different tools are provided below. Please see the documentation for their different usages. For example, sigma clipping, which is a common way to estimate the background of an image, can be performed with the sigma_clip() function. By default, the function returns a masked array, a type of Numpy array used for handling missing or invalid entries. Masked arrays retain the original data but also store another boolean array of the same shape where True indicates that the value is masked. Most Numpy ufuncs will understand masked arrays and treat them appropriately. For example, consider the following dataset with a clear outlier:

>>> import numpy as np
>>> from astropy.stats import sigma_clip
>>> x = np.array([1, 0, 0, 1, 99, 0, 0, 1, 0])


The mean is skewed by the outlier:

>>> x.mean()
np.float64(11.333333333333334)


Sigma-clipping (3 sigma by default) returns a masked array, and so functions like mean will ignore the outlier:

>>> clipped = sigma_clip(x)
>>> clipped
masked_array(data=[1, 0, 0, 1, --, 0, 0, 1, 0],
mask=[False, False, False, False,  True, False, False, False,
False],
fill_value=999999)
>>> clipped.mean()
np.float64(0.375)


If you need to access the original data directly, you can use the data property. Combined with the mask property, you can get the original outliers, or the values that were not clipped:

>>> outliers = clipped.data[clipped.mask]
>>> outliers
array([99])
>>> valid = clipped.data[~clipped.mask]
>>> valid
array([1, 0, 0, 1, 0, 0, 1, 0])


For more information on masked arrays, including see the numpy.ma module.

### Examples#

To estimate the background of an image:

>>> data = [1, 5, 6, 8, 100, 5, 3, 2]
>>> data_clipped = stats.sigma_clip(data, sigma=2, maxiters=5)
>>> data_clipped
masked_array(data=[1, 5, 6, 8, --, 5, 3, 2],
mask=[False, False, False, False,  True, False, False, False],
fill_value=999999)
>>> np.mean(data_clipped)
np.float64(4.285714285714286)


Alternatively, the SigmaClip class provides an object-oriented interface to sigma clipping, which also returns a masked array by default:

>>> sigclip = stats.SigmaClip(sigma=2, maxiters=5)
>>> sigclip(data)
masked_array(data=[1, 5, 6, 8, --, 5, 3, 2],
mask=[False, False, False, False,  True, False, False, False],
fill_value=999999)


In addition, there are also several convenience functions for making the calculation of statistics even more convenient. For example, sigma_clipped_stats() will return the mean, median, and standard deviation of a sigma-clipped array:

>>> stats.sigma_clipped_stats(data, sigma=2, maxiters=5)
(np.float64(4.285714285714286), np.float64(5.0), np.float64(2.249716535431946))


There are also tools for calculating robust statistics, sampling the data, circular statistics, confidence limits, spatial statistics, and adaptive histograms.

Most tools are fairly self-contained, and include relevant examples in their docstrings.

## Using astropy.stats#

More detailed information on using the package is provided on separate pages, listed below.

## Constants#

The astropy.stats package defines two constants useful for converting between Gaussian sigma and full width at half maximum (FWHM):

gaussian_sigma_to_fwhm#

Factor with which to multiply Gaussian 1-sigma standard deviation to convert it to full width at half maximum (FWHM).

>>> from astropy.stats import gaussian_sigma_to_fwhm
>>> gaussian_sigma_to_fwhm
2.3548200450309493

gaussian_fwhm_to_sigma#

Factor with which to multiply Gaussian full width at half maximum (FWHM) to convert it to 1-sigma standard deviation.

>>> from astropy.stats import gaussian_fwhm_to_sigma
>>> gaussian_fwhm_to_sigma
0.42466090014400953


## Performance Tips#

If you are finding sigma clipping to be slow, and if you have not already done so, consider installing the bottleneck package, which will speed up some of the internal computations. In addition, if you are using standard functions for cenfunc and/or stdfunc, make sure you specify these as strings rather than passing a NumPy function — that is, use:

>>> sigma_clip(array, cenfunc='median')


>>> sigma_clip(array, cenfunc=np.nanmedian)