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
>
>