Asked by arif rabbani
on 5 May 2013

i have performed extreme distribution to a data set.I intended to draw 95% confidence interval in the probability plot.The following codding was used:

WL= [ 6.79 6.89 6.38 5.67 6.91 7.66 6.41 6.04 6.41 5.55 6.93 5.67 6.69 6.51 6.32 7.33 6.43 6.52 6.13 6.72 6.7 7.78 6.18 5.41 6.94 6.51 6.02 5.94 6.08 ]; % observed water level data pd = fitdist(WL','ev'); % Creates a ProbDistUnivParam object by fitting the data to a extreme value distribution. ci = paramci(pd); % This function calculates the values of the parameters based on a certain confidence interval. Here the by default the confidence interval is 95 percent probplot(WL); h=probplot(gca,@(WL,x,y)evcdf(WL,x,y),[ci(1,1),ci(1,2)]); set(h,'color','r','linestyle','-') t= probplot(gca,@(WL,x,y)evcdf(WL,x,y),[ci(2,1),ci(2,2)]); set(t,'color','g','linestyle','-.')

But there is a problem in confidence interval line(green one) which deviated in a wrong way.there might be some error in ci(confidence interval) boundary but i could not fix it.please help me out.

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Answer by José-Luis
on 6 May 2013

Edited by José-Luis
on 7 May 2013

Accepted answer

I thing there are two problems with your approach. One is that you could simplify your code like this:

WL= [ 6.79 6.89 6.38 5.67 6.91 7.66 6.41 6.04 6.41 5.55 6.93 5.67 6.69 6.51 6.32 7.33 6.43 6.52 6.13 6.72 6.7 7.78 6.18 5.41 6.94 6.51 6.02 5.94 6.08 ]; % observed water level data pd = fitdist(WL','ev'); % Creates a ProbDistUnivParam object by fitting the data to a extreme value distribution. ci = paramci(pd); % This function calculates the values of the parameters based on a certain confidence interval. Here the by default the confidence interval is 95 percent probplot('extreme value' , WL); h=probplot(gca,@evcdf,[ci(1,1),ci(1,2)]); set(h,'color','r','linestyle','-') t= probplot(gca,@evcdf,[ci(2,1),ci(2,2)]); set(t,'color','g','linestyle','-.')

Note that probplot is by default a normal distribution. You should specify whenever you use a different distribution. Secondly, what makes you think that there is an error with what you did? This is not curve fitting, you are comparing cdfs. There is no reason why the parameters you obtained from *ci* will give you two cdf's "above" and "below" the best fit cdf. You probably expect that from vanilla curve fitting. But these are not the "curves" you fitted to, these are cdf's.

Please take a look at the cdf's, as they might help you understand better:

WL= [ 6.79 6.89 6.38 5.67 6.91 7.66 6.41 6.04 6.41 5.55 6.93 5.67 6.69 6.51 6.32 7.33 6.43 6.52 6.13 6.72 6.7 7.78 6.18 5.41 6.94 6.51 6.02 5.94 6.08 ]; % observed water level data pd = fitdist(WL','ev'); % Creates a ProbDistUnivParam object by fitting the data to a extreme value distribution. ci = paramci(pd); % This function calculates the values of the parameters based on a certain confidence interval. Here the by default the confidence interval is 95 percent evBest = @(x) evcdf(x,pd.Params(1),pd.Params(2)); evLow = @(x) evcdf(x,ci(1,1),ci(1,2)); evHigh = @(x) evcdf(x,ci(2,1),ci(2,2));

fplot(evBest,[0 10],'k--'); hold on fplot(evLow,[0 10],'r-'); fplot(evHigh,[0 10],'b-');

arif rabbani
on 7 May 2013

The graph i intend to draw is like the graph showed below in the link:

thank you .

Tom Lane
on 10 May 2013

Answer by Tom Lane
on 10 May 2013

You are right that probplot has no feature for including confidence bounds.

The cdf method for the probability distribution object, I believe, can return lower and upper bounds as its second and third outputs for some distributions. It appears not to be documented, but is visible in generated code from dfittool so I would expect it to be supported.

Try this:

x = evrnd(10,2,40,1); p = fitdist(x,'ev') xx = linspace(icdf(p,.001),icdf(p,.999)); n = length(x); [f,flo,fhi] = cdf(p,xx); plot(sort(x),icdf(p,(1:n)/(n+1)),'rx',xx,icdf(p,f),... 'b-',xx,icdf(p,flo),'r:',xx,icdf(p,fhi),'r:')

This code computes bounds on the cdf, then converts the cdf using the inverse cdf so the fit plots as a straight line. The resulting plot shows the observed points as red X's and the fitted quantiles as the blue line. This is basically the probability plot, except that the y axis is labeled with quantile values instead of probability values. Perhaps this will be adequate. You could try to label the y axis with the probability values if that is important.

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