1723 Views 11 Replies Latest reply: Apr 7, 2012 4:10 PM by StuartBruff
165 posts since
Nov 5, 2011
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## What is a Frequency Chart, how to generate?

Moderator: Please delete if this type inquiry is not appropriate. Just trying to tap into the vast knowledge base here......

Have an assignement coming up where we will have to classify a set of data via a frequency chart.

I intend to implement in Mathcad, but have never heard about this frequency scale on the chart. The Professor gave no references we could look up on the origin of the chart. He said he would give us instructions in Excel at a later date on how to build such a chart. Am curious if anybody here would be familiar with such a frequency plot and implementation in Mathcad?

From what I could see in class, the frequency scale on the absicissa starts at .01, with 50 in the middle of the axis and ends at 99.99. I'd say on this scale 1 would be about a third of the way to 50 and 20 at 2/3 of the way, then symetry on the other side of 50 with 70 at 1/3rd of the way to 99.99 and 99 at 2/3rd of the way.

Anybody familiar with this "frequency" scale and its origin/background and how to generate a graph based on this scale from a set of data pairs?

Thank you.

• 150 posts since
Jun 25, 2007
Currently Being Moderated
Mar 26, 2012 11:15 PM (in response to ptc-4310378)
Re: What is a Frequency Chart, how to generate?

Many types of data distributions fit a "Gaussian" (also called "Normal") distribution, which sounds like what you are looking for.  If the outcome of an event has only two possibilities (such as heads or tails from a coin toss), it is called the "Binomial" distribution.

You can use the "Histogram" function in Mathcad to "bin" the data and see how it fits.

Any introductory book on statistics should cover this in sufficient detail.

• 150 posts since
Jun 25, 2007
Currently Being Moderated
Apr 2, 2012 12:45 PM (in response to ptc-4310378)
Re: What is a Frequency Chart, how to generate?

Mathcad does not specifically have a graph like this as far as I know, but you can make one that has similar characteristics by using the Weibull distribution functions.  The x-axis will be in standard deviations from the mean, but you can easily convert this to percentiles using these functions.

There is a sample worksheet (a.k.a. "Quicksheet") showing how normal distribution functions are related to percentiles under the "Normal Distribution" topic.  It should not be too much effort to convert this into a Weibull plot.

• 150 posts since
Jun 25, 2007
Currently Being Moderated
Apr 3, 2012 11:04 AM (in response to ptc-4310378)
Re: What is a Frequency Chart, how to generate?

The example graph you linked to had the word "Weibull" on it, which I thought referred to the distribution being a Weibull one.

But it may be a "normal" distribution based on other material on the internet.

I agree with you on the cookie cutter approach for now.  Hold you nose and use Excel.

• 150 posts since
Jun 25, 2007
Currently Being Moderated
Apr 3, 2012 3:45 PM (in response to ptc-4310378)
Re: What is a Frequency Chart, how to generate?

In Excel, NORMINV(probability, mean, std_dev) returns the inverse of the normal cumulative distribution for the specified mean and standard deviation.

The equivalent in MC15 is qnorm().  I also see that MC15 has "log normal distribution" functions, which might be useful for your task.  I've never used them.

3.719 is the (approximate) number of standard deviations to the right of the mean where the 99.99th percentile begins for a normal distribution.

BTW - One of my pet peeves (among many) is professors or bosses who have just enough skill or facility in Excel to make a complicated "template" file that they absolutely insist you use, not because it's so good, which it is not, but because they think it somehow shows the world how clever or computer savvy they are.  I've seen huge multi-column spreadsheets of data that Mathcad could have chewed up and spit out in a few lines.  Ugh!

• 150 posts since
Jun 25, 2007
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Apr 3, 2012 6:39 PM (in response to ptc-4310378)
Re: What is a Frequency Chart, how to generate?
There is some savvy behind this Excel Chart. Never saw anything quite like that before. The Professor had even hid away the worksheet (adequately named "Charts Lines")  where the scale is referenced from, to make it look like magic! After   "unhiding" the sheet, it becomes easier to understand how it is done.

I'm sometimes amazed at the spreadsheet gymanastics that people go through in Excel.

It becomes incredibly dense with calculations referencing cells that reference other cells that reference other cells that...and so on.  In some cases, I'm not sure if the answer is right or not, but by that point I have a headache and don't care anymore.

At least with Mathcad there is a good chance of figuring out what another person has done.

• 4,864 posts since
Jun 1, 2007
Currently Being Moderated
Apr 7, 2012 4:10 PM (in response to MichaelH)
Re: What is a Frequency Chart, how to generate?

MichaelH wrote:

I'm sometimes amazed at the spreadsheet gymanastics that people go through in Excel.

It becomes incredibly dense with calculations referencing cells that reference other cells that reference other cells that...and so on.  In some cases, I'm not sure if the answer is right or not, but by that point I have a headache and don't care anymore.

At least with Mathcad there is a good chance of figuring out what another person has done.

Tell me about it, already.    I checked out one Excel worksheet that had been used to determine the choice of numerical integrator for a 6-dof simulation program.   The big clue to the fact that there might have been errors in the worksheet was that the 4th order Runge-Kutta integrator performed worse than the simple Euler.   Tracking the error down wasn't easy because of the cell referencing scheme used.

I don't even trust my own Excel worksheets, let alone anybody else's!

Stuart

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