Suzanne, there is nothing wrong with un-equal block lengths, but see
the discussion below for a point about efficiency.
Unequal Block Sizes - from SPM posting in 2005
>I am planning a blocked fMRI design to investigate interactions
>between implicit and explicit memory processes.
>Is there any reason against using blocks with unequal durations,
>i.e. shorter block for implicit encoding and longer blocks for
>Because there are the opportunity to use mixed (block and event)
>designs in fMRI experiments, I do not see any problems in a fMRI
>design with unequal block durations.
>Are there someone who has experience with unequal block durations
>in fMRI experiments?
>Thank`s a lot for recommendations in advance
I don't think there's anything wrong with it _per se_.
The disadvantage with unequal block durations is one of statistical
efficiency. It's the same issue as with a classic 2-sample t-test: if
one sample is much larger than the other, the efficiency is lower than if
the two samples are of the same size (all things being equal, viz, the
total sample size (sum of the two samples) being the same).
I assume you don't want to make the two blocks the same duration because
of some kind of psychological or behavioral design constraint.
One issue that you haven't directly asked about is that it's best if the
block types mixed up in time, e.g. ABABABAB..., not AAAABBBB, and not AB.
Otherwise you get confounded with low frequency noise (slow drifts).
On 2/14/17, suzanne whiston <[log in to unmask]> wrote:
> Dear SPM experts
> I have a question about bolck-design. Does the blocks' time fix? By this I
> mean that, can a block-design in the following form?
> (20s impulse) (15s rest) (30s impulse) (40s rest) (10s impulse) (8s rest)
> Best Regards