[R] QQ plots and boxcox

[John Fox]

Dear Erin,

What you characterize as "snaky" might also be called bimodal and
short-tailed. Try, e.g., plot(density(residuals(dog1.aov))) to see the
bimodality more clearly. A Box-Cox transformation can correct skewness, but
won't help here.

I hope this helps,
 John

------------------------------
John Fox, Professor
Department of Sociology
McMaster University
Hamilton, Ontario, Canada
web: socserv.mcmaster.ca/jfox


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On
> Behalf Of Erin Hodgess
> Sent: December-02-08 4:28 PM
> To: r-help at r-project.org
> Subject: [R] QQ plots and boxcox
> 
> Dear R People:
> 
> In the DASL library, there is a story about hot dogs.
> 
> Here are the data:
> Beef	186	495
> Beef	181	477
> Beef	176	425
> Beef	149	322
> Beef	184	482
> Beef	190	587
> Beef	158	370
> Beef	139	322
> Beef	175	479
> Beef	148	375
> Beef	152	330
> Beef	111	300
> Beef	141	386
> Beef	153	401
> Beef	190	645
> Beef	157	440
> Beef	131	317
> Beef	149	319
> Beef	135	298
> Beef	132	253
> Meat	173	458
> Meat	191	506
> Meat	182	473
> Meat	190	545
> Meat	172	496
> Meat	147	360
> Meat	146	387
> Meat	139	386
> Meat	175	507
> Meat	136	393
> Meat	179	405
> Meat	153	372
> Meat	107	144
> Meat	195	511
> Meat	135	405
> Meat	140	428
> Meat	138	339
> Poultry	129	430
> Poultry	132	375
> Poultry	102	396
> Poultry	106	383
> Poultry	94	387
> Poultry	102	542
> Poultry	87	359
> Poultry	99	357
> Poultry	107	528
> Poultry	113	513
> Poultry	135	426
> Poultry	142	513
> Poultry	86	358
> Poultry	143	581
> Poultry	152	588
> Poultry	146	522
> Poultry	144	545
> 
> 
> Here is my work:
> > dog1.df <- read.table(file="dog1.dat",as.is=F,header=F,
> + col.names=c("type","cal","sodium"))
> > dog1.aov <- aov(cal~type,data=dog1.df)
> > summary(dog1.aov)
>             Df  Sum Sq Mean Sq F value    Pr(>F)
> type         2 17692.2  8846.1  16.074 3.862e-06 ***
> Residuals   51 28067.1   550.3
> ---
> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
> > TukeyHSD(dog1.aov)
>   Tukey multiple comparisons of means
>     95% family-wise confidence level
> 
> Fit: aov(formula = cal ~ type, data = dog1.df)
> 
> $type
>                    diff       lwr       upr     p adj
> Meat-Beef      1.855882 -16.82550  20.53726 0.9688129
> Poultry-Beef -38.085294 -56.76667 -19.40391 0.0000277
> Poultry-Meat -39.941176 -59.36515 -20.51720 0.0000239
> 
> > plot(dog1.aov)
> 
> When I look at the QQ plot, it's quite "snaky".
> I thought that doing a transformation (based on boxcox) would help
> with the snakiness.
> 
> However, when I re-run with the transformation, it's still snaky.
> 
> Any suggestions, please?
> 
> Thanks,
> Sincerely,
> Erin
> 
> 
> --
> Erin Hodgess
> Associate Professor
> Department of Computer and Mathematical Sciences
> University of Houston - Downtown
> mailto: erinm.hodgess at gmail.com
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.