Mark
In the results section, you should use the same threshold for 4 subjects
or 8 subjects. The p value you get in the results table is your Pw (see
below). This is usually p<0.05 corrected OR p<0.001 uncorrected if you
have an a priori hypotheses (which you say you don't).
Pc (see below) is the threshold you use when specifying which contrasts
you are using. If you specify 4 contrasts, a threshold of 0.1778 for
each contrast (subject) should be <0.001 uncorrected for the conjunction of
all 4 effects (Pw). Likewise if you specify 8 contrasts, a threshold of
0.4217 for each contrast (subject) should be significant at p<0.001
uncorrected for the conjunction of all 8 effects.
I'm assuming that you are using SPM99 because you said you were using a
subject x condition interaction design.
If you are using SPM97, then Pc corresponds to the masking and Pw
corresponds to the threshold for the conjunction when specifying the
results and in the results table.
Is this clearer?
Cathy
>Cathy,
>
>Many thanks for your speedy reply to my query. However, your answer has
>raised
>further questions in my mind.
>
>When I run the conjunction analysis I obviously get the results table with
>areas of activation and associated p values at the voxel and cluster
>level.. My
>question is therefore at what level of p value do the results become
>"significant" in a conjunction. I.e. if the corrected p value for an
>activation is p=0.24 and the uncorrected p<0.001 is it "valid" to report this
>as a "significant" result because it has survived a conjunction of 12 subjects
>at Karl's suggested threshold (from your reply) even though I had no a priori
>hypothesis for this location?
>
>Many thanks again.
>
>Mark
>
>Cathy Price wrote:
>
>> Hello Mark
>> According to Karl, the following heuristic can be applied.
>> The threshold you need to chose for each subject (Pc) should equal the
>> threshold you want for the conjunction (Pw), when Pc is raised to the power
>> of n.
>>
>> So if the uncorrected threshold you want for the conjunction is p<0.001
>> then the threshold for each subject is 0.4217 for 8 subjects (approximately
>> what you used)
>> and 0.1778 for 4 subjects (higher than you used).
>>
>> i.e.
>> >> 0.001^(1/4) = 0.1778
>>
>> Hopefully this stricter threshold with the lower P value will give you more
>> sensible results
>> for the n=4 group.
>> Cathy
>>
>> >Dear SPMers,
>> >
>> >I have a data set of 12 subjects who had 12 run water PET scans. The
>> >subjects split into 2 groups (n=8 & 4 for the groups). I would like to
>> >see which areas of activation are common to the subjects in each group.
>> >I therefore want to do conjunction analyses for each group of subjects.
>> >My question concerns appropriate thresholding (p values) for the results
>> >from these conjunctions.
>> >
>> >I have entered the data using the subject x condition interaction design
>> >for PET studies with each subject having 6 scans under each condition.
>> >I have then done a separate contrast for each subject (-1 1). I then
>> >obviously do a conjunction of these individual contrasts including the 4
>> >or 8 subjects I am interested in. I have no a priori hypotheses of
>> >particular areas that I am expecting differences so suspect that I
>> >should be using corrected p values.
>> >
>> >I have specified low thresholds for the contrast conjunctions (eg.
>> >p<0.5) which for 8 subjects give fairly discrete activations in sensible
>> >places, but for 4 subjects most of the brain is pale gray. Intuitively
>> >I can understand that the fewer subjects I include in a conjunction the
>> >more likely activations will be common in the conjunction. What I do
>> >not understand is some way to quantify this so that I do not make type I
>> >or type II errors interpreting the conjunctions.
>> >
>> >Help on this would be appreciated.
>> >
>> >Mark
>> >
>> >
>> >========================
>> >Dr. Mark Daglish
>> >Clinical Research Fellow
>> >Psychopharmacology Unit
>> >University of Bristol
>> >========================
>
>--
>========================
>Dr. Mark Daglish
>Clinical Research Fellow
>Psychopharmacology Unit
>========================
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