[GRASS-user] r.regression.line... differences with regression function LINEST in openoffice.org-calc (?)

[Markus Neteler]

My suggestion is again to rewrite it a C program (not sure, based
on r.univar code?). I found a couple of bugs in the script and I am
(as you) not sure that it does what it promises to do).

Markus


Nikos Alexandris wrote:
> 
> I have submitted some comments upon the documentation of
> the "r.regression.line" script in
> http://wald.intevation.org/tracker/?func=detail&aid=552&group_id=21&atid=207
> 
> Now I have 2 questions.
> 
> Apologies if I miss some obvious things but I am confused!
> 
> Here it goes:
> 
> 1. I don't understand why (in lines 84 and 85 in the
> r.regression.line script) "sumsqX=sumsqX/tot" and
> "sumsqY=sumsqY/tot" ?
> 
> 2. I can't understand the differences in the following... :
> I created two raster maps with the same MASK, each
> containing only 6 pixels with the following values:
> 
> mapA: 326 641 1336 2020 3197 3484
> mapB: 432 850  931 1956 2582 2622
> 
> For mapA "r.univar" gives:
> n: 6
> minimum: 326
> maximum: 3484
> range: 3158
> mean: 1834
> mean of absolute values: 1834
> standard deviation: 1194.44
> variance: 1.4267e+06
> variation coefficient: 65.1278 %
> sum: 11004)
> 
> For mapB... :
> n: 6
> minimum: 432
> maximum: 2622
> range: 2190
> mean: 1562.17
> mean of absolute values: 1562.17
> standard deviation: 866.145
> variance: 750207
> variation coefficient: 55.4451 %
> sum: 9373)
> 
> In openoffice calc (some of) the respective results are:
> 
> For mapA:
> MIN: 326
> MAX: 3484
> AVERAGE: 1834
> STDEV: 1308,45
> VAR: 1,71E+06
> SUM: 11004
> 
> For mapB:
> MIN: 432
> MAX: 2622
> AVERAGE: 156217
> STDEV: 948,81
> VAR: 9,00E+05
> SUM: 9373
> 
> 
> Based on r.regression.line I get
> 
> for map1=mapA and map2=mapB:
> 
> a  b  R  N  F medX  sdX  medY  sdY
> 0.000458151 0.809242 0.99157 1021726 -0.98321 0.01077
> 5.3038 0.00917369 4.32854
> 
> and
> 
> for map1=mapB and map2=mapB:
> a  b  R  N  F medX  sdX  medY  sdY
> -0.000375823 1.21498 0.99157 1021726 -0.98321 0.00917369
> 4.32854 0.01077 5.3038
> 
> "R" is Pearson's correlation coefficient (as correctly
> defined in the script "r.regression.line" in line 83 but
> wrongly expressed as "sumXY - sumX*sumY/tot" in the
> print-out in line 101).
> 
> In openoffice-calc I get for these:
> MapA    MapB
> 326	432	1,350792		Slope m	
> 641	850	0,138835		standard error of the slope	
> 1336	931	0,959458		RSQ (Square of "r")
> 2020	1956	94,662802	4,000000	F value from the variance
> analysis    std error of regression for Y
> 3197	2582	8213134,001379		sum of squared deviation of
> estimated Y values from their linear mean	
> 3484	2622				
> 
> or
> 
> MapB    MapB
> 432	326	0,710293		Slope m	
> 850	641	0,073004		standard error of the slope	
> 931	1336	0,959458		RSQ	
> 1956	2020	94,662802	4,000000	F value from the variance
> analysis    std error of regression for Y
> 2582	3197	4318750,949062		sum of squared deviation of
> estimated Y values from their linear mean	
> 2622	3484
> 
> 
> (How is really r.regression.line functioning? Trying to
> interpret the script is not that easy for me since I lack
> of some basics in scripting)
> 
> 
> Thank you,
> 
> Nikos.
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> 
> 

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