Trials getting flipped

[Shivani R]

Hi,

I am using E Prime to develop the experiments. In this experiment, the
subject sees a fixation mark followed by an image. The subject's task
is to determine if the image was a target or distractor. Basically
there are 14 different trial lists in this experiment and all of them
consists of 100 trials each, 50 targets and 50 distractors. Each time
a trial list is selected at random and its 100 stimuli are presented
on the screen randomly such that no more than 3 targets or 3
distractors occur consecutively. To achieve this randomization, I am
using a sample script provided by pstnet.com in the samples section.
I am printing the image name, its type and corresponding correct
answer in the output window. The problem I am facing is some of the
trials are getting flipped. I mean a distractor image is displayed on
the screen but its type is taken as target and the correct answer is
taken as 1. Actually if a distractor image is selected, its type
should be distractor and also its correct answer should be 5. One
important thing I noticed in here is that a target image is never
taken as a distractor image, I mean a target image is never getting
flipped. When the first trial list is selected, this flipping problem
is not there usually. It starts from 2nd or 3rd trial list, with every
3 - 4 trials out of 100 trials getting flipped (distractor being
treated as a target). Towards the end of the experiment, say 10th or
11th trial list, this problem is much more pronounced and every 6 - 7
trials are getting flipped out of 100 trials.

I tried running the experiment 15 times, and each time this flipping
problem was there. Distractor got treated as a target but never the
other way round and also the flipping is not because of same images
each time, the images getting flipped keep on changing. One other
important point that I noticed here is when a particular trial list is
selected at the 1st position, there are no flips, but if I run the
experiment again and if this list got selected at some other position
say 9th or 11th or something towards the end of the experiment, there
were flips in the trial list.

If I remove the randomization script, there are no flips in the entire
experiment. So basically I know that this flipping is occurring
because of the randomization script. I tried whatever I could but to
no use. I have been struggling with this problem for last one month.
It would be really nice if some one can give some solution to this
problem.


Any help is really appreciated.


Regards
Shivani

-- 
You received this message because you are subscribed to the Google Groups "E-Prime" group.
To post to this group, send email to e-prime at googlegroups.com.
To unsubscribe from this group, send email to e-prime+unsubscribe at googlegroups.com.
For more options, visit this group at http://groups.google.com/group/e-prime?hl=en.