How to calculate r-squared in exponential fitting?

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Inês Rodrigues 2017-7-17

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https://ww2.mathworks.cn/matlabcentral/answers/349130-how-to-calculate-r-squared-in-exponential-fitting

Hello. So I'm trying to find the r-squared in an exponencial fitting. Having x and y, I did first: Y = log(y); X = x; s_X = sum(X); s_Y = sum(Y); s_X_2 = sum(X.^2); s_XY = sum(X.*Y); N = length(X); B = (s_XY - (s_X_2 * (s_Y/s_X)))/(s_X-(s_X_2*N/s_X)); A = (s_Y - N*B)/s_X; C = exp(B);

My model is then y = C*exp(A*x). I checked in Excel and A and C are the same.

Then I did yfit = C*exp(A*X), and finally R2 = 1- (norm(yfit - Y)/norm(Y- mean(Y)))^2

However, it outputs me a negative value, of course not the same as excel

Could you help me?