[GRASS-dev] [GRASS GIS] #1668: r.regression.line F-test incorrect

[GRASS GIS]

#1668: r.regression.line F-test incorrect
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 Reporter:  cmbarton     |       Owner:  grass-dev@…              
     Type:  defect       |      Status:  new                      
 Priority:  normal       |   Milestone:  7.0.0                    
Component:  Raster       |     Version:  unspecified              
 Keywords:               |    Platform:  Unspecified              
      Cpu:  Unspecified  |  
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Comment(by mlennert):

 Replying to [comment:2 cmbarton]:
 > These DO give VERY different values. For slope vs. elevation in my
 Spearfish example, the current equation gives a value of -0.2320019531,
 while the revised equation gives a value of 1.02410689008565E-006.
 >
 >
 http://www.weibull.com/DOEWeb/hypothesis_tests_in_simple_linear_regression.htm
 defines
 > F = SumSq Regression /( SumSq Errors / (count - 2))

 Yup, sorry, in my proposal another set of parentheses was missing. It
 should be:

 F = R * R / ((1 - R * R) / (count - 2));

 where

 R*R = SumSq Regression
 1-R*R = SumSq Errors

 which in your example gives you a value for F = 788781, i.e. a probability
 that there is a relationship so close to one that most software will
 probably just give you 1 after rounding.

 Again, I can commit this, but would like to have someone more versed in
 statistics confirm.

 Moritz

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Ticket URL: <http://trac.osgeo.org/grass/ticket/1668#comment:3>
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