R help - Nov 2010 - conditional mean between two data frames with different levels

Hi guys, I have two data frames: one referred to 2008 and one to 2009. Their
structure is identical except for the different data in them. 
I need to create a vector alfa of the same length of the dataframe 2009 and
fill each element with the mean of 2008$var1 conditional to the subgroup
indicated by a factor variable in 2009$var2.
In this case it would be easy to use the function
alfa[i]<-ave(2008$var1,2008$var2==2009$var2[i],FUN=mean).
The problem is that 2008$var2 and 2009$var2 contain both 20 levels each but
only 18 of them are shared. So for those 18 I need to find the result that
I`d get applying the above formula (which in any case doesnt work if the
levels are not identical in the two data frames colmns) and for those two
different levels in 2009$var2 to use the average of the whole column
2008$var2.
Anybody has some ideas? Please help me...
Hope it`s clear enough what I need.
Thanks!
alberto 
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Hi Alberto,

It would help if you could provide a small example.  I might break the
problem down into three parts:  1) create a vector that has the final
subgroupings you want 2) find the conditional means by subgroup 3)
replicate the means as needed.  My first guess would be start with:


"==" or "%in%" to compare or find levels from 2008 %in% 2009

by(Data, GroupingVar, FUN = mean)

This is probably not the best way, but since "GroupingVar" should be a
factor, I would be tempted to do:

tmp <- (factor(GroupingVar, levels = levels(GroupingVar), labels
ResultsofByCall)
YourMeans <- as.numeric(levels(tmp))[tmp]

as a way to map the means back to their appropriate subcondition
replicated as many times as necessary.  I'm sure you will get more
detailed help if you can post a bit of sample data.

HTH,

Josh

On Thu, Nov 18, 2010 at 8:10 AM, albechan <alberto.casetta at satt.biz>
wrote:
>
> Hi guys, I have two data frames: one referred to 2008 and one to 2009.
Their
> structure is identical except for the different data in them.
> I need to create a vector alfa of the same length of the dataframe 2009 and
> fill each element with the mean of 2008$var1 conditional to the subgroup
> indicated by a factor variable in 2009$var2.
> In this case it would be easy to use the function
> alfa[i]<-ave(2008$var1,2008$var2==2009$var2[i],FUN=mean).
> The problem is that 2008$var2 and 2009$var2 contain both 20 levels each but
> only 18 of them are shared. So for those 18 I need to find the result that
> I`d get applying the above formula (which in any case doesnt work if the
> levels are not identical in the two data frames colmns) and for those two
> different levels in 2009$var2 to use the average of the whole column
> 2008$var2.
> Anybody has some ideas? Please help me...
> Hope it`s clear enough what I need.
> Thanks!
> alberto
> --
> View this message in context:
http://r.789695.n4.nabble.com/conditional-mean-between-two-data-frames-with-different-levels-tp3049010p3049010.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help at r-project.org mailing list
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/

Thank you very much Josh, I guess you`re right.
So this is an example:
data frame 1 has 2 columns and 10 rows. The first column is "score" a
variable indicating the number of goals scored by a football team
score<-c(1,2,0,2,1,1,3,2,1,0), column 2 contains the football "teams
" where
teams<-c(a,b,c,d,e,a,b,c,d,e).
Data frame 2 has the following variables: score<-c(2,3,1,0,0,0,4,2,1,2) and
"teams"<-c(b,c,d,e,f,b,c,d,e,f).
What I need is to create a vector "alfa"<-numeric(10) where the
first
element contains the mean of the number of goals scored by team b in the
previous season, the second element contains the mean of the number of goals
scored by team c in the previous season and so on. In correspondance of team
f, the average of the whole score vector of the previous season.
alfa should be (2.5, 1, 1.5, 0.5, 1.3, 2.5, 1, 1.5, 0.5, 1.3)
The problem arises because "f" doesnt appear in the first data frame
as it
replaced "a".
Hope the issue is more understandable now.
Thanks a lot!
alberto 
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