[R] [OT] 1 vs 2-way anova technical question

Mon Nov 21 17:37:43 CET 2011

the way I interpret the problem (and I may be wrong here, I don't think you 
have been particularly clear with your question) is that you are trying to 
make a factorial anova where you are trying to explain "R" as a result of 
A,B,C and D, and their interaction terms. so using A*B*C*D.
what you should consider is the error family of your data (poisson, 
binomial...) and use: model<-glm(R~A*B*C*D) and then simplify your model.
I suggest reading chapter 7 in Crawleys "Statistics: an introduction using 
R." and combined with the statistical knowledge you have learnt on one of 
your courses you should hopefully find the answer. Perhaps you could also 
speak to someone within the course you are registered to and some statistics 
focussed forums - it tends to annoy some people on here when they find a 
stats question on their R mailing list, obviously they don't have a delete 
button...

Good luck.
Rob

-----Original Message----- 
From: Giovanni Azua
Sent: Monday, November 21, 2011 4:59 PM
To: r-help at r-project.org
Subject: Re: [R] [OT] 1 vs 2-way anova technical question

Hello Bert,

Thank you for taking the time to try to answer.

1) I know this, however if one is interested in only interaction between two 
specific factors then in R one uses I(A*B*C) meaning 3-way anova for that 
and not the implicit 2-ways that would otherwise be computed.

2) True, but it fails.

3) No, I don't have any factors with one level, I never said that. It would 
not be a 2^k experiment otherwise, my OP states this clearly, this is a 2^k 
experimental design ___2___

4) this is only your judgmental attitude that many people unfortunately have 
in some of these lists, focussing on ad-hominem judgements or even attacks 
to try to prove their superiority without actually answering nor adding any 
value to the question at hand. I have taken many graduate courses in 
subjects that have all Statistics in the title and passed all of them. 
However, as an experienced Software Engineer working for more than 10 years 
in the field, I can tell you that there is a huge difference between solving 
toy problems to implementing real-life complex projects.  Same rules apply 
here, one thing is the toy examples one finds in R books and course 
exercises and another totally different story is the real life data I am 
trying to model. I'm a student in the quantitative part and learning, so I 
do have some gaps, I am curious and trying to learn and I think there is no 
shame in that. If this makes you upset maybe you should ask to split the 
list in two or more: "Advanc!
ed-PhD-black-belt-10th-dan-in-Statistics-and-R level" list and "newbies" 
list.

Best regards,
Giovanni

On Nov 21, 2011, at 3:55 PM, Bert Gunter wrote:

> Giovanni:
>
> 1. Please read ?formula and/or An Introduction to R for how to specify
> linear models in R.
>
> 2. Correct specification of what you want (if I understand correctly) is
> log(R) ~ A*B + C + D
>
> 3. ... which presumably will also fail because some of your factors
> have only one level, which means that you cannot use them in your
> model.
>
> 4. ... which, in turn, suggests you don't know what your doing
> statistically and should seek local assistance, especially in trying
> to interpret a fit to an unbalanced model (you can't do it as you
> probably think you can).
>
> I should say in your defense that posts on this list indicate that
> point 4 is a widely shared problem among posters here.
>
> Cheers,
> Bert
>
> On Mon, Nov 21, 2011 at 5:02 AM, Giovanni Azua <bravegag at gmail.com> wrote:
>> Hello,
>>
>> Couple of clarifications:
>> - A,B,C,D are factors and I am also interested in possible interactions 
>> but the model that comes out from aov R~A*B*C*D violates the model 
>> assumptions
>> - My 2^k is unbalanced i.e. missing data and an additional level I also 
>> include in one of the factors i.e. C
>> - I was referring in the OP to the 4-way interactions and not 2-way, I'm 
>> sorry for my confusion.
>> - I tried to create an aov model with less interactions this way but I 
>> get the following error:
>>
>> model.aov <- aov(log(R)~A+B+I(A*B)+C+D,data=throughput)
>> Error in `contrasts<-`(`*tmp*`, value = "contr.treatment") :
>>  contrasts can be applied only to factors with 2 or more levels
>> In addition: Warning message:
>> In Ops.factor(A, B) : * not meaningful for factors
>>
>> Here I was trying to say: do a one-way anova except for the A and B 
>> factors for which I would like to get their 2-way interactions ...
>>
>> Thanks in advance,
>> Best regards,
>> Giovanni
>>
>> On Nov 21, 2011, at 12:04 PM, Giovanni Azua wrote:
>>
>>> Hello,
>>>
>>> I know there is plenty of people in this group who can give me a good 
>>> answer :)
>>>
>>> I have a 2^k model where k=4 like this:
>>> Model 1) R~A*B*C*D
>>>
>>> If I use the "*" in R among all elements it means to me to explore all 
>>> interactions and include them in the model i.e. I think this would be 
>>> the so called 2-way anova. However, if I do this, it leads to model 
>>> violations i.e. the homoscedasticity is violated, the normality 
>>> assumption of the sample errors i.e. residuals is violated etc. I tried 
>>> correcting the issues using different standard transformations: log, 
>>> sqrt, Box-Cox forms etc but none really improve the result. In this case 
>>> even though the model assumptions do not hold, some of the interactions 
>>> are found to significatively influence the response variable. But then 
>>> shall I trust the results of this Model 1) given that the assumptions do 
>>> not hold?
>>>
>>> Then I try this other model where I exclude the interactions (is this 
>>> the 1-way anova?):
>>> Model 2) R~A+B+C+D
>>>
>>> In this one the model assumptions hold except the existence of some 
>>> outliers and a slightly heavy tail in the QQ-plot.
>>>
>>> Given that the assumptions for Model 1) do not hold, I assume I should 
>>> ignore the results altogether for Model 1) or? or instead can I safely 
>>> use the Sum Sq. of Model 1) to get my table of percent of variations?
>>>
>>> This to me was a bit counter-intuitive since I assumed that if there was 
>>> collinearity among factors (and there is e.g. I(A*B*C)) the Model 1) and 
>>> I included those interactions, my model would be more accurate ... ok 
>>> this turned into a brand new topic of model selection but I am mostly 
>>> interested in the question: if model is violated can I or must I not use 
>>> the results e.g. Sum Sqr for that model?
>>>
>>> Can anyone advice please?
>>>
>>> btw I have bought most books on R and statistical analysis. I have 
>>> researched them all and the ANOVA coverage is very shallow in most of 
>>> them specially in the R-sy ones, they just offer a slightly pimped up 
>>> version of the R-help.
>>>
>>> I am also unofficially following a course on ANOVA from the university I 
>>> am registered in and most examples are too simplistic and either the 
>>> assumptions just hold easily or the assumptions don't hold and nothing 
>>> happens.
>>>
>>> Thanks in advance,
>>> Best regards,
>>> Giovanni
>>>
>>
>>
>>        [[alternative HTML version deleted]]
>>
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>
>
>
> -- 
>
> Bert Gunter
> Genentech Nonclinical Biostatistics
>
> Internal Contact Info:
> Phone: 467-7374
> Website:
> http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm

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