occasional skips

[Caren Frosch]

I'm running an experiment where I have divided a list into four blocks, 
that is, the list consists of 160 trials and after 40 trials it leaves 
the list to run another procedure which consists of one slide where 
participants are told whether or not to generate random numbers during 
the next block (actually one of 3 procedures: 'taskproc' 'notaskproc', 
'endproc'). It's all been running fine. But I have found that for some 
participants it occasionally skips this step and they therefore end up 
doing one big block (consisting of 80 trials). Has anyone experienced 
this kind of thing before and do you have any suggestions as to what it 
might be? Could it be a hardware problem (e.g. the keyboard)?

Any suggestions would be much appreciated as I just can't figure out why 
it's doing it. When I run through the experiment myself it's always 
fine. It's also not possible that participants are accidentally hitting 
the key that moves the slide on as I deliberately chose a key away from 
the keys they are using to respond to the trials (and I don't tell them 
which key it is).

Thank you,
Caren