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niexianling铜虫 (初入文坛)
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[求助]
matlab中std2函数计算矩阵的标准差时，分母是除以了n-1还是n？
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matlab中std2函数计算矩阵的标准差时，分母是除以了n-1还是n？ 这是如下代码 function s = std2(a) %STD2 Standard deviation of matrix elements. % B = STD2(A) computes the standard deviation of the values in % A. % % Class Support % ------------- % A can be numeric or logical. B is a scalar of class double. % % Example % ------- % I = imread('liftingbody.png'); % val = std2(I) % % See also CORR2, MEAN2, MEAN, STD. % Copyright 1992-2010 The MathWorks, Inc. % $Revision: 5.19.4.6 $ $Date: 2010/09/13 16:14:04 $ % validate that our input is valid for the IMHIST optimization fast_data_type = isa(a,'logical') || isa(a,'int8') || isa(a,'uint8') || ... isa(a,'uint16') || isa(a,'int16'); % only use IMHIST for images of sufficient size big_enough = numel(a) > 300000; if fast_data_type && isequal(ndims(a),2) && ~issparse(a) && big_enough % compute histogram if islogical(a) num_bins = 2; else data_type = class(a); num_bins = double(intmax(data_type)) - double(intmin(data_type)) + 1; end [bin_counts bin_values] = imhist(a, num_bins); % compute standard deviation total_pixels = numel(a); sum_of_pixels = sum(bin_counts .* bin_values); mean_pixel = sum_of_pixels / total_pixels; bin_value_offsets = bin_values - mean_pixel; bin_value_offsets_sqrd = bin_value_offsets .^ 2; offset_summation = sum( bin_counts .* bin_value_offsets_sqrd); s = sqrt(offset_summation / total_pixels); else % use simple implementation if ~isa(a,'double') a = double(a); end s = std(a(  );end |
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csgt0: 金币+1, 谢谢 2013-06-28 11:32:57
感谢参与，应助指数 +1
csgt0: 金币+1, 谢谢 2013-06-28 11:32:57
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默认的是N-1吧，以下是MATLAB 帮助文档的说明： STD Standard deviation. For vectors, Y = STD(X) returns the standard deviation. For matrices, Y is a row vector containing the standard deviation of each column. For N-D arrays, STD operates along the first non-singleton dimension of X. STD normalizes Y by (N-1), where N is the sample size. This is the sqrt of an unbiased estimator of the variance of the population from which X is drawn, as long as X consists of independent, identically distributed samples. Y = STD(X,1) normalizes by N and produces the square root of the second moment of the sample about its mean. STD(X,0) is the same as STD(X). Y = STD(X,FLAG,DIM) takes the standard deviation along the dimension DIM of X. Pass in FLAG==0 to use the default normalization by N-1, or 1 to use N. |
2楼2013-06-27 10:34:09
niexianling
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