why am i getting zero slope?

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aine gormley 2018-12-6

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https://ww2.mathworks.cn/matlabcentral/answers/434203-why-am-i-getting-zero-slope

评论： Stephen23 2018-12-6

I calculate linear regression (temperature change (y) over 30 years (x)) using a large dataset (producing >55000 slopes). 62% of the slopes are exactly zero, which makes no sense.

Also, although no error comes up, I am getting a warning 'x is rank deficient to machine precision', presumably this is because of the zeros. The regress function, I understood, manages nan values by excluding them. Should I tell it to treat zeros as nan, or is that too much of an assumption?

% Read in the necessary variables
lat = ncread('file.nc', 'lat');
lon = ncread('file.nc', 'lon');
time = ncread('file.nc', 'time');
TXx = double(ncread('file.nc', 'TXx'));
% Create storage for regression
M = numel(lon);
N = numel(lat);
slope = zeros(M, N);
intercept = zeros(M, N);
T = numel(time);
% Regress each lat/lon location
for i = 1 : M
    for j = 1 : N
        % Get all time instances of each lat/lon location
        y = squeeze(TXx(i, j, :));
        
        % Create regression problem and solve
        x = [ones(T, 1) time];
        c = regress(y, x);
        intercept(i, j) = c(1);
        slope(i, j) = c(2);
    end
end

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aine gormley 2018-12-6

hi, thanks for your message, sorry -kind of a newbie here - but how would i do a representative sub-sample?

Stephen23 2018-12-6

"but how would i do a representative sub-sample?"

You pick enough of your data so that it shows the behavior that you describe. Start by selecting one single set of x, y, and c arrays where this situation occurs, and upload them here in one .mat file.

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