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Thanks. I really really appreciate your help:) Mark 2013/1/17 Jason Steffener <[log in to unmask]> > Yes, that is very high. > You can compare mediators using simple contrasts (i.e. subtract a1*b > and a2*b and test with bootstrapping. > > Jason > > On Thu, Jan 17, 2013 at 1:40 PM, Tseng Mark <[log in to unmask]> wrote: > > Thank you again, Jason. > > > > The ab/c ratio is 65.8%. Is this very high? > > > > By the way, how can I determine which mediator is most important (or at > > least more important than another one) if I have 2 mediators? Supposed we > > did collect M' data If we conducted 2 mediation analyses, say (a) X--Y > > mediated by M and (b) X--Y mediated by M', and the ab/c ratio in (a) is > > higher than that in (b), we can then say that, as a mediator, M is more > > important than M'? > > > > Best, > > > > Mark > > > > 2013/1/17 Jason Steffener <[log in to unmask]> > >> > >> Just to be clear: > >> > >> c' is the direct effect of X on Y > >> c is the total effect of X on Y > >> ab is the indirect effect of X on Y > >> > >> c = c' + ab > >> > >> The test of whether age-related differences in behavior are > >> significantly related to age-related differences in fMRI activity is > >> the test of whether ab is significant. The best way to test this is > >> with BCa confidence intervals on a bootstrap test. > >> > >> If ab is significant then yes you can say that X has and significant > >> indirect effect on Y via M. You can then look at the ratio of ab/c to > >> determine how much of the age effect is via M. I do not think that you > >> have any grounds to state that M is the most important mediator unless > >> the ab/c ratio is very high. > >> > >> Jason > >> > >> > >> > >> On Thu, Jan 17, 2013 at 12:59 PM, Tseng Mark <[log in to unmask]> wrote: > >> > Sorry a correction: c' is not zero. c' = –0.029. > >> > > >> > > >> > 2013/1/17 Jason Steffener <[log in to unmask]> > >> >> > >> >> Dear Mark, > >> >> I think you should avoid the discussion of full versus partial > >> >> altogether in relation to other unmeasured variables because this > >> >> distinction is based solely on a significance threshold. Paths c and > >> >> c' are not needed for their to be a significant indirect effect (ab) > >> >> and it is the significance of your indirect effect that is of > interest > >> >> to you. This is another reason for avoiding the full/partial > >> >> discussion. > >> >> > >> >> > >> >> To address your specific question, is c' zero? Or is it just > >> >> not-significant? If it does not go to zero after accounting for M, > >> >> then there is still an effect of X on Y in the presence of M. It may > >> >> be that M' may be that effect. So I do not think you have any basis > >> >> for saying that M' has no effect on X, M or Y especially since M' is > >> >> not measured. > >> >> > >> >> best, > >> >> Jason. > >> >> > >> >> On Thu, Jan 17, 2013 at 12:20 PM, Mark <[log in to unmask]> wrote: > >> >> > Results showed that path > >> > > >> > > > > > >