How to convert a factor column into a numeric one?

Thanks for your help.

As far as your question below is concerned, the data frame arose as a 
result of some data cleaning on an original data frame, which was 
changed into a table, modified, and changed back to a data frame:

ttcrmean<- as.table(by(ngbe[,'Log10'], 
list(Time=ngbe$Time,Temp=ngbe$Temp,Conc=ngbe$Conc,Repl=ngbe$Replicate),
   mean))
for (k in 1:3) {  #fix-up time zeroes
   for (l in 1:5) { #replicates
     t0val<- ttcrmean[1,3,k,l]
     for (j in 1:4) {  #temps
       ttcrmean[1,j,k,l]<- t0val
     } #j
   } #l
} #i
df<- na.omit(as.data.frame(ttcrmean))
colnames(df)[5]<- 'Log10'

Hi Robert,
<snip>
I would also look into *why* those numeric columns are being stored as
factors in the first place.  If you are reading the data in with
read.table() or one of its wrapper functions (like read.csv), then it
would be better to preempt the storage as a factor altogether rather
than converting back to numeric.  For example, perhaps something is
being used to indicate missing data that R does not recognize (e.g.,
SAS uses ".").  Specifying na.strings = ".", would fix this.  See
?read.table for some of the options available.
<snip>

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Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral at lcfltd.com
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