On Sat, 20 May 2000, Manni Heumann wrote:
> We are doing research in visual perception. To measure subjects ability
> two perceive certain stimuli we computed d' or ds. But these measures
> only provide information about the ability to detect one signal.
I don't know what d' or ds are (or perhaps I do not recognize them in
this rather spare context); so it's difficult to guess whether there
simply do not exist analogues of them in multiple-signal contexts, or
there are different ways of defining analogues and you haven't managed
to settle on a suitable way.
> We are in the process of designing a new experiment and we would like
> to know, whether subjects can discriminate 3 different stimuli.
Do you really want to know "whether" (for which the answer must surely be
"Yes", at some (possibly uninteresting) level of discourse)?
Or do you want to know "how well" or "how reliably" the stimuli can be
discriminated?
> Unfortunately we do not see a way to use signal detection theory here.
> The problem is, that you can tell a hit from some error, but what would
> be a false alarm?
Another kind of error, presumably. With only 1 stimulus of interest, you
have two kinds of "hits" (yes, the stimulus is present; no, it isn't)
and two kinds of "misses" or errors (no it isn't, but in fact it was;
yes it is but in fact it wasn't, or a "false alarm"). With 3 stimuli of
interest you have more kinds of "hits" and even more kinds of "misses".
One simple conceptualization (A, B, C the three stimuli of interest; D
some other stimulus (a distractor); N no stimulus at all, or only
background noise; + = a hit, x = a miss):
Perception (what the subject reports)
A B C D N
Stimulus A + x x x x
(what is B x + x x x
presented) C x x + x x
D x x x + x
N x x x x +
There are 5 kinds of hits and 20 distinguishable kinds of misses.
Are you interested in distinguishing among all of these severally, or do
you consider several of these 25 situations as equivalent (for, say,
purposes of computing an index of accuracy or of confusability or ...)?
Do you want to know about asymmetries (e.g., A is reported when B is
present more frequently than B is reported when A is present)?
Do you want to know what kinds of distractors most (or least) interfere
with ability to detect A, B, C?
Do you want to know what subjects report when more than one of the
target stimuli are present? (In that case you want a larger table, with
rows and columns for the combinations AB, AC, BC, ABC, and possibly some
combinations with various Ds.)
> Does anyone know of a statistical procedure that would allow us to
> compute one or several indices, that could be used to measure subjects
> performance? Of course this would also influence the question(s) we ask
> in the experiments and probably the number of trials we need per
> subject and stimulus. But since we are still in the design stage, this
> would not be a great difficulty.
Perhaps this conceptualization will help you to approach the various
possibilities. I am out of time now...
-- DFB.
------------------------------------------------------------------------
Donald F. Burrill [EMAIL PROTECTED]
348 Hyde Hall, Plymouth State College, [EMAIL PROTECTED]
MSC #29, Plymouth, NH 03264 603-535-2597
184 Nashua Road, Bedford, NH 03110 603-471-7128
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