Re: identification task...negative d' values.

[Erick Gallun <gallun@xxxxxx>]

Negative values simply mean that the listener consistently used the 
wrong label.  Quite reasonable in an identification task, I 
suppose.  If the values are small (less than the variability of the 
measure) I would chalk it up to random variability.  If they are 
large, then it is a meaningful result that should be interpreted.  I 
wonder, though, how are you calculating d' from three phoneme 
labels?  Feel free to contact me off-list.

Erick Gallun
Postdoctoral Fellow
Hearing Research Center
Boston University


At 01:22 PM 5/2/2006, silvadmr wrote:
> Dear list,
>
> I have some few small negative values of d' obtained from an 
> identification task in witch subjects label vowel sounds from a 
> continuum with tree phoneme labels. Generally, the graphs and tables 
> in the literature never show negative values of d' ( i think 
> it  occurs here and there).
> The interpretation of such values from an identification task of 
> that kind must not be the same as the interpretation of negative 
> values obtained in a yes-no experiment...cause in the case of 
> indentification we have "different sounds" rather then a "signal 
> stimulus" and a "signal plus noise stimulus".
>
> What is the usually adopted procedure?
> Should I take the absolute values (given the nature of the 
> identification task)?...should I convert them to 0? should I keep 
> them as negative values?
>
> Thanks!