I am using a two-interval force choice (2IFC) task to estimate detection threshold and choose an appropriate signal level for my following experiment, by estimating sensitivity ($d'$) for each signal level.
Since it is a 2IFC task with signal equally distributed on two intervals, I expect close-to-zero response bias and plan to estimate $d'$ directly from proportion correct. After running experiment on 3 subjects (one complete 2500 trials, two complete 1250 trials), two of them show slight response bias (ratio of response to interval 1 vs. 2 is 0.91 and 1.12), but one subject shows large response bias (ratio of response to interval 1 vs. 2 is 0.53).
Here is my question:
For the subject with large response bias, is it still valid to estimate $d'$ from proportion correct anymore? And is it still valid to estimate $d'$ directly from hit rate and false-alarm rate?
If I hope to let this subject to continue participating in this experiment, how should I let him/her to lower response bias? Directly telling him/her "You have pressed key #1 too often, press key #2 more when you are not sure"? Is it going to interfere the experiment?
For future naïve participants, should I let them know do not bias toward one response too much before the start of experiment, or should I tell them only when they have made too much bias? If the latter case, how much bias is considered to be "too much"?