physical understanding in t-stat and p-value In swat cup

[gangaram maharjan]

Dear all,

I ran the swat cup with simulation number of 1565 for the 14 parameters. Now i would like to rank the sensitive parameters. I check the file Sensitivity.xls file in the folder "L:\sed_cup\Tcup1.Sufi2.SwatCup\SUFI2.OUT" where i found t-State and P-value for corresponding parameters. With little understanding of t-Stat and P-Value and as explained in swat cup manual, I choose highest sensitive parameter as 1st in sensitive ranking that correspond to highest absolute t-stat. Let me know the sensitivity Rank that i assign highest sensitive parameter as 1st rank for highest absolute t-Stat is correct assumption or not. Here i have included the parameters with sensitive rank and respective t-stat and P-value as the screen shot presented below. 

However i read swat cup manual (version 4) for better understanding for t-Stat and P-value. I would be thankful for brief explanation of t-Stat and P-Value estimated in SWAT-cup. 

I have attached goal.txt for latin hypercube generated parameters and corresponding objective funciont

wish you nice day

Sincerely yours
--

Ganga Ram Maharjan

Phd. Student,

University of Bayreuth, Universitaetsstrasse 30
NW1 Room 7.001 Soil Physics,

(Research: Land use change, sediment transport,water quality,crop production and  environmental trade off)

[Abbaspour, Karim]

You did OK! You can find definitions in any basic statistical books.

Best, Karim

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[Abbaspour, Karim]

I use a multiple regression analysis to get the statistics of parameter sensitivity. The t-stat is the coefficient of a parameter divided by its standard error. It is a measure of the precision with which the regression coefficient is measured. If a coefficient is “large” compared to its standard error, then it is probably different from 0 and the parameter is sensitive. What is “large”? You could compare the t-stat of a parameter with the values in the Student's t-distribution table to determine the p-value, which is the number that you really need to be looking at. The Student's t-distribution (you find at the end of most statistics book) describes how the mean of a sample with a certain number of observations is expected to behave. The p-value for each term tests the null hypothesis that the coefficient is equal to zero (no effect). A low p-value (< 0.05) indicates that you can reject the null hypothesis. In other words, a predictor that has a low p-value is likely to be a meaningful addition to your model because changes in the predictor's value are related to changes in the response variable. Conversely, a larger p-value suggests that changes in the predictor are not associated with changes in the response. So that parameter is not very sensitive. A p-value of < 0.05 is the generally accepted point at which to reject the null hypothesis (i.e., the coefficient of that parameter is different from 0). With a p-value of 0.05, there is only a 5% chance that results you are seeing would have come up in a random distribution, so you can say with a 95% probability of being correct that the variable is having some effect.

Hope this is more clear.

-------------------------------------------------
Dr. K.C. Abbaspour
Eawag, Swiss Federal Institute for Aquatic Science and Technology
Ueberlandstr. 133, P.O. Box 611, 8600 Duebendorf, Switzerland
email: abba...@eawag.ch
phone: +41 58 856 5359
fax: +41 58 856 5375
http://www.eawag.ch/index_EN

 

 

From: swat...@googlegroups.com [mailto:swat...@googlegroups.com] On Behalf Of gangaram maharjan
Sent: Wednesday, April 29, 2015 10:49 AM
To: swat...@googlegroups.com; swat...@googlegroups.com
Subject: physical understanding in t-stat and p-value In swat cup

 

Dear all,

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[Arcadius Degan]

Hi Mrs Ganga and Karim

I ran the swat cup with simulation number of 100 for the 13 parameters, but now i can't open global sensitivity, neither one-a-time sensitivity. when i go to the folder of mySwatCup/Sufi2project, in the folder of SUFI2OUT there is no Sensitivity.xls file.How to poceed to get this file?

Thank to answer 

[Shiva]

Thank you very much Karim for the detailed explaination.

Diva

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[Shiva]

Thank you Sir for asking this doubt, I too find it difficult in interpretation!

Diva