var(a, axis=None, dtype=None, out=None, ddof=0, keepdims=False)¶
Compute the variance along the specified axis.
LAX-backend implementation of
var(). Original docstring below.
Returns the variance of the array elements, a measure of the spread of a distribution. The variance is computed for the flattened array by default, otherwise over the specified axis.
a (array_like) – Array containing numbers whose variance is desired. If a is not an array, a conversion is attempted.
dtype (data-type, optional) – Type to use in computing the variance. For arrays of integer type the default is float64; for arrays of float types it is the same as the array type.
out (ndarray, optional) – Alternate output array in which to place the result. It must have the same shape as the expected output, but the type is cast if necessary.
ddof (int, optional) – “Delta Degrees of Freedom”: the divisor used in the calculation is
N - ddof, where
Nrepresents the number of elements. By default ddof is zero.
keepdims (bool, optional) – If this is set to True, the axes which are reduced are left in the result as dimensions with size one. With this option, the result will broadcast correctly against the input array.
variance – If
out=None, returns a new array containing the variance; otherwise, a reference to the output array is returned.
- Return type
ndarray, see dtype parameter above
The variance is the average of the squared deviations from the mean, i.e.,
var = mean(abs(x - x.mean())**2).
The mean is normally calculated as
x.sum() / N, where
N = len(x). If, however, ddof is specified, the divisor
N - ddofis used instead. In standard statistical practice,
ddof=1provides an unbiased estimator of the variance of a hypothetical infinite population.
ddof=0provides a maximum likelihood estimate of the variance for normally distributed variables.
Note that for complex numbers, the absolute value is taken before squaring, so that the result is always real and nonnegative.
For floating-point input, the variance is computed using the same precision the input has. Depending on the input data, this can cause the results to be inaccurate, especially for float32 (see example below). Specifying a higher-accuracy accumulator using the
dtypekeyword can alleviate this issue.
>>> a = np.array([[1, 2], [3, 4]]) >>> np.var(a) 1.25 >>> np.var(a, axis=0) array([1., 1.]) >>> np.var(a, axis=1) array([0.25, 0.25])
In single precision, var() can be inaccurate:
>>> a = np.zeros((2, 512*512), dtype=np.float32) >>> a[0, :] = 1.0 >>> a[1, :] = 0.1 >>> np.var(a) 0.20250003
Computing the variance in float64 is more accurate:
>>> np.var(a, dtype=np.float64) 0.20249999932944759 # may vary >>> ((1-0.55)**2 + (0.1-0.55)**2)/2 0.2025