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Jun 9, 2020 7:30 PM
(649 views)

Hi,

Is there an option to view the probability plot when making a log-normal fit with a distribution analysis?

Thanks

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Hi,

From the red hotspot of beside the name of your column in the Distribution platform choose "Continuous Fit -->Fit lognormal." Then, from red hotspot beside "Fitted Lognormal Distribution," choose "Diagnostic Plots." You then have the option of fitting the P-P and/or Q-Q plots. Is that what you're looking for?

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@HadleyMyers provided you with a solution. However, if your lognormal distribution has a large mean and/or variance, the scaling on the QQ plot usually does not have the familiar custom probability plot that scales the Y axes like custom probability paper (yes I am showing my age, things we used pre graphical calculator/computer days).

I suggest you right click on the column of data you're fitting, select **Transform** and use **Log**, then **Continuous Fit > Fit Normal**.

Below is a screenshot of ** Analyze > Distribution > Continuous Fit > Fit Normal** of 100 random lognormal(4,2) values transformed with Log. The table with script is attached.

Another suggestsion is to use the QQ plot that Hadley suggested and click on the Y axis of he QQ plot and change Linear to Log.

4 REPLIES 4

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Hi,

From the red hotspot of beside the name of your column in the Distribution platform choose "Continuous Fit -->Fit lognormal." Then, from red hotspot beside "Fitted Lognormal Distribution," choose "Diagnostic Plots." You then have the option of fitting the P-P and/or Q-Q plots. Is that what you're looking for?

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@HadleyMyers provided you with a solution. However, if your lognormal distribution has a large mean and/or variance, the scaling on the QQ plot usually does not have the familiar custom probability plot that scales the Y axes like custom probability paper (yes I am showing my age, things we used pre graphical calculator/computer days).

I suggest you right click on the column of data you're fitting, select **Transform** and use **Log**, then **Continuous Fit > Fit Normal**.

Below is a screenshot of ** Analyze > Distribution > Continuous Fit > Fit Normal** of 100 random lognormal(4,2) values transformed with Log. The table with script is attached.

Another suggestsion is to use the QQ plot that Hadley suggested and click on the Y axis of he QQ plot and change Linear to Log.

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Re: Probability plot for log-normal distribution fitting

Hi gzmorgan0,

I agree, making the log transformation is a great way to get the intervals on the plot too. I did this before but was also wondering about getting a plot for the log fit. The PP/QQ do a good job.

Thanks for the help.

I agree, making the log transformation is a great way to get the intervals on the plot too. I did this before but was also wondering about getting a plot for the log fit. The PP/QQ do a good job.

Thanks for the help.

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Re: Probability plot for log-normal distribution fitting

Hi Hadley,

That is what I was looking for but without the confidence interval. But not a big deal.

Thanks for showing me around.

That is what I was looking for but without the confidence interval. But not a big deal.

Thanks for showing me around.