Emilio- 
One possible way to do this is to create a new factor and put the levels
in the order you want them in.
 
For example:
related.differences$header2 <- factor(related.differences$header,
levels=c("spontaneous recovery", "negative reinforcer", ... etc. )
 
where you want spontaneous recovery to be the first level, negative is
the second, and so forth.
 
Cheers,
Andrea Johnson
 

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