(From Bruce Abbott [950222.0945 EST])
Bill Powers (950221.2210 MST)
I got my analysis program to run, finally, and took some data on me. It
was an interesting experience. I kept thinking things like "This is
ridiculous, I have to keep bouncing my gaze back and forth between the
two lines, and how can anyone say when the lines are "equal", and I'm
really doing terribly at this, I'll be lucky to get a correlation of
0.5." So here are the results"x-y x/y
k 0.0291 8.672
r -62 0.75
RMS 13.7 15.3
Corr 0.9545 0.9529
That's the feeling I got. Correlations of 0.95+, some "terrible," huh? You
might want to try a sine-wave disturbance. It's easier to track (if not too
fast) and gives interesting results.
Is there any utility to looking at the rates of change of the two
lines? I have a procedure that does this and, as might be
expected, these are also correlated, although not as well as the
absolute lengths are.It probably won't gain much -- it will mostly amplify the high-frequency
noise which doesn't correlate with much.
Yes, but I have a procedure that removes much of this by doing a running
average of difference scores over some specifiable span. Longer spans
filter out more of the high-frequency noise. If I use a span of around
60-90 I get a derivative curve that is fairly smooth and does a fairly good
job of following the derivative of the sine wave (i.e., cosine) when I use
the sine-wave disturbance. It's easy to see from the plot where I was
accelerating the mouse to catch up, overshooting, etc.
When we've gone as far as we care to with the T-illusion, I'd like to
try a different one, the "plumber's illusion" as I know it.> > /
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/ | |This would be compensatory tracking; the slanted line on the left stays
still and the line on the right is disturbed straight up and down. The
object, of course, is to move the right-hand line up and down to make it
line up with the left-hand line. Mabe the central "wall" should be a
filled rectangle.
Excellent idea. Plumber's illusion? This one is known in psychology as the
Poggendorff illusion, but I can see how it could have gotten the name you
know it by. Perhaps Poggendorff moonlighted as a plumber during semester
breaks? (;->
Cheers,
Bruce