# Plotting Probability Exceedance using the Probability Plot Graph in Minitab

B

#### big ec

Hi

Does anyone know how to plot probability exceedance using the probability plot graph in minitab. I'm talking here about plotting the probability of exceeding a certain value whereas it seems that minitab is only able to plot the probability of a value being less than a certain value.

This would be used for plotting the probability of a flow rate for a river exceeding a certain value i.e. a low flow rate would have a high probability value with a high flow rate having a low probability value.

Many thanks for any help offered!

#### Statistical Steven

##### Statistician
Staff member
Super Moderator
Re: Probability plot of exceedance

Hi

Does anyone know how to plot probability exceedance using the probability plot graph in minitab. I'm talking here about plotting the probability of exceeding a certain value whereas it seems that minitab is only able to plot the probability of a value being less than a certain value.

This would be used for plotting the probability of a flow rate for a river exceeding a certain value i.e. a low flow rate would have a high probability value with a high flow rate having a low probability value.

Many thanks for any help offered!
I might be slow or not understanding your question correctly, but the probability of being greater than a value is 1-probability of being less than that value.

B

#### big ec

Re: Probability plot of exceedance

Thanks for your help Steve - however what I'm looking for is an option in minitab to allow me to produce a probability plot of this - essentially taking 1 column of data and then showing on a plot the probability of a figure in that column being exceeded.

I can produce a probablity plot where the probability of a new value being less than a value in the column is plotted but not the probability of a new value being greater

Need to get this done for a master degree assignment on Friday!

#### Tim Folkerts

Super Moderator
It is not an elegant solution but I think this will do what you need ...

Once you have the standard graph made, copy the graph into an image processing application (the free MS Paint on windows machines will do). Once there, select just the graph (not the titles, numbers, etc) and "Flip Vertical". This should make the graph slope down and the biggest value will be down near 1% instead of up near 99%.

I didn't see any good way to do it within Minitab.

(I did also notice that if you hover the mouse over the lines on the probability plot, it will show the actual data. I can't seem to cut & paste, but you could in principle copy the numbers, change all the probabilities to 1-p, and then replot the results (perhaps in Excel) .

A

#### Allattar

If I understand you right you could use the calc tools.

CALC>PROBABILITY DISTRIBUTIONS>NORMAL

Create a column of x-values, I recommend patterned data tools from calc.

For example -3 to +3 steps of 0.1.
Then use the calc probability distributions to obtain the cumulative probability for each value of x.
Then use the calculator to take 1-the cumulative probability.

Head to graphs and scatterplots, plotting with a connect line and plot x against 1-cumulative probability. To make it tidier turn the plotted points off displaying just the connect line.

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