I have the following problem:

I have this data

What I have tried so far (with R):

Chi-squared test for outlier
X-squared = 2.0888, p-value = 0.1484
alternative hypothesis: highest value 90 is an outlier

Dixon test for outliers
Q = 0.7143, p-value = 0.1544
alternative hypothesis: highest value 90 is an outlier

I also tried an Grubbs test (I found a tool online at: http://www.graphpad.com/quickcalcs/Grubbs1.cfm )
I got a critical Z value of 1.45 (1.48 would be significant).

I want to get the value of 90 out by an outlier test. I am not too experienced in outlier testing (I have no experience :) ). So could anybody name me a test, that is more "sensitive" concerning outliers so that th value of 90 is thrown out of my data? Thanks a lot!

Yours Hubert
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Ambassador to the humans
I don't know too much about tests for checking if something is an outlier but I'm guessing you won't find one that shows that 90 is an outlier when you only have 4 data points.

But you might be able to increase power by making certain distributional assumptions? How did you get this data?
But you might be able to increase power by making certain distributional assumptions? How did you get this data?
Thanks so far! I got it by quantitative PCR. This method is normally very reproducible and I am sure that it is an Outlier. Usually values just differ by +/- 10 or something like this...
But I can't say "I feel, this is an outlier, so I leaf it out". Therefore I have to proof it somehow...

Thanks a lot!



TS Contributor

I am not very familiar with the tests you quoted, but I would like to point out a method that is based on median and median absolute deviation (MAD).

Basically, it is a robust method which try to circumvent the issue called "masking" (outliers tending to mask themselves inflating the standard deviation).

A data point is flagged as outlier if:
value - median > 2*(MedianAbsoluteDeviation/0,6745)

You will find info about this method in this book: http://www.amazon.co.uk/Fundamental...1663557?ie=UTF8&s=books&qid=1261934308&sr=1-1

I implemented this method in a free Excel template that you can download from my website. See this previous post for a previous discussion on this topic and for download location http://www.talkstats.com/showthread.php?t=10461

By the way, I tried it on your dataset and 90 was declared outlier.

Hope this helps,

Hallo gianmarco!

Thanks so much for that. It really helped me alot so far.

I have downloaded your file and tried to find out, where you have filled in my data, but I doesn't find the right place. Could you maybe name to sheet and the area (eg "A2" in sheet "CI-Median_Table"). Thank you so much!

For those who will also read this postings later. The book is also available online (google books). You can find it here: http://books.google.at/books?id=uUN...num=1&ved=0CCoQ6wEwAA#v=onepage&q=MAD&f=false

On page 33 is a detailed description of the methode.



TS Contributor
you can download my template and open the file. Then, go to the worksheet sample 1 (or sample 2, it is the same) and enter your values in the first left column. Choose the appropriate method to detect outlier: there is a specific menù for this. Please, read the guideline that comes along with the template.

I hope you manage to use the template.

For any further help, feel free to write to' me.

All the best,