numpy.ma.var¶
- numpy.ma.var(self, axis=None, dtype=None, out=None, ddof=0) = <numpy.ma.core._frommethod instance at 0x4243f92c>¶
Compute the variance along the specified axis.
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.
Parameters: a : array_like
Array containing numbers whose variance is desired. If a is not an array, a conversion is attempted.
axis : int, optional
Axis along which the variance is computed. The default is to compute the variance of the flattened array.
dtype : data-type, optional
Type to use in computing the variance. For arrays of integer type the default is float32; 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 N represents 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 original arr.
Returns: variance : ndarray, see dtype parameter above
If out=None, returns a new array containing the variance; otherwise, a reference to the output array is returned.
Notes
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 - ddof is used instead. In standard statistical practice, ddof=1 provides an unbiased estimator of the variance of a hypothetical infinite population. ddof=0 provides 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 dtype keyword can alleviate this issue.
Examples
>>> 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) 0.20405951142311096
Computing the variance in float64 is more accurate:
>>> np.var(a, dtype=np.float64) 0.20249999932997387 >>> ((1-0.55)**2 + (0.1-0.55)**2)/2 0.20250000000000001