Hi,

I have x, y vector data where x = some independent variable of interest and y = cumulative probability. I know the resulting curve represents a lognormal cdf but I'm having trouble finding a way to find the location and scale parameters that correspond to it.

My initial thought was to simply take the cdf, convert it to a pdf by taking p(ii) = y(ii+1) - y(ii), and then use the frequency option of lognfit to find the parameters. I do not get the correct result from this though and was wondering if anyone else had any ideas. Example code is below. Thanks!

if true

end

X = 1:200;

Y = logncdf(X,4.5,0.1);

for ii = 1:length(X)-1

P(ii) = Y(ii+1)-Y(ii);

end

P(200) = 1 - Y(end);

fit = lognfit(X,[],[],P)

The location parameter I get from this example is correct, but the scale parameter is 0.