[EMAIL PROTECTED] (Donald F. Burrill) wrote:
>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.
>
d' and ds are signal detection theory measures of sensitivity. Unfortunately,
I don't know of any analogues, but I guess, that there aren't.
>> 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?
>
We expect, that subjects cannot discriminate between the stimuli, or at least
that is what we hope for. So a dichotomous measure would be enough, although
it should be rather sensitive.
>> 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".
The false alarms are critical. The advantage of d' or ds is that they are not
affected by response bias. Suppose a subject answered the question, whether
the signal was present, consistently with "Yes!". That would result in 100%
hits. On the other hand you'd also get 100% false alarms. Simplifying, you can
say that d'=hits-falseAlarms. To me the distinction of a false alarm and a
miss is vital, but seems to break down in the multi signal context.
--
Manni
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