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RegisterArbitrary (and unjustified) cut-off of r² [RSABE / ABEL]
Dear Helmut,
Small add-on: adj. r² method could be misleading in "the more (datapoints) the better"! Considering the PK of Azithromycin, "the less the better" could be considered, because there are (at least?) 3 elimination phases for Azi (uptake into white blood cells, rapid distribution into tissue... would resemble Anscombe2), and a very long t1/2, depending (!) on the timepoints used for calculation. As terminal elimination needs to be calculated and the adj r² method from previous study took mostly 3 to 5 points, but sometimes also 12 (!), should the timepoints be limited (to 4 to 3), to reflect PK? What if one concentration looks to be an analytical mistake (?), that confounds t1/2 in such a way that the slope increases...? Where to put the cut-off for adj r²?
Thanks a lot in advance
FI
❝ In other words, an \(\small{r^2}\) of 0.6486 from five data points denotes the same ‘quality of fit’ than an \(\small{r^2}\) of 0.9755 from three. Searching the forum I get the impression that you (AB) are not alone with a cut-off of 0.80. Justification: nil.
❝ BTW, visual inspection of fits is mandatory... Don’t trust in numbers alone. A classical example is Anscombe’s quartet.2
Small add-on: adj. r² method could be misleading in "the more (datapoints) the better"! Considering the PK of Azithromycin, "the less the better" could be considered, because there are (at least?) 3 elimination phases for Azi (uptake into white blood cells, rapid distribution into tissue... would resemble Anscombe2), and a very long t1/2, depending (!) on the timepoints used for calculation. As terminal elimination needs to be calculated and the adj r² method from previous study took mostly 3 to 5 points, but sometimes also 12 (!), should the timepoints be limited (to 4 to 3), to reflect PK? What if one concentration looks to be an analytical mistake (?), that confounds t1/2 in such a way that the slope increases...? Where to put the cut-off for adj r²?
Thanks a lot in advance
FI
Complete thread:
- SAS error in 3 way ref replicate study AB 2012-05-04 15:13 [RSABE / ABEL]
![[Mix]](https://static.bebac.at/img/mix.png "open in Mix view")
- SAS error in 3 way ref replicate study jag009 2012-05-04 15:36
- SAS error in 3 way ref replicate study AB 2012-05-05 06:45
- FDA's ABE code and partial replicate design d_labes 2012-05-06 10:10
- Arbitrary (and unjustified) cut-off of r² Helmut 2012-05-06 13:29
- Anscombe quartet d_labes 2012-05-07 08:48
- Anscombe quartet in R Helmut 2012-05-07 11:15
- Anscombe quartet in R AB 2012-05-07 12:07
- Anscombe quartet in R Helmut 2012-05-07 11:15
- Arbitrary (and unjustified) cut-off of r²FI 2012-10-08 10:39
- Predominant half life; exclusions Helmut 2012-10-08 13:45
- Excluding time points for lambdaZ d_labes 2012-10-09 09:33
- Analytical variability Helmut 2012-10-09 14:09
- Answer machine d_labes 2012-10-09 15:15
- Well done! Helmut 2012-10-09 19:43
- Answer machine d_labes 2012-10-09 15:15
- Analytical variability Helmut 2012-10-09 14:09
- Excluding time points for lambdaZ d_labes 2012-10-09 09:33
- Predominant half life; exclusions Helmut 2012-10-08 13:45
- Anscombe quartet d_labes 2012-05-07 08:48
- Arbitrary (and unjustified) cut-off of r² Helmut 2012-05-06 13:29
- SAS error in 3 way ref replicate study jag009 2012-05-04 15:36
Ing. Helmut Schütz