Subjects are tasked to press a key every time a letter works except when the letter X appears. In such case, subjects should NOT press any keys.
Bias and sensitivity are well explained here https://www.frontiersin.org/articles/10.3389/fpsyg.2020.00073/full or here: https://rstudio-pubs-static.s3.amazonaws.com/371840_5694e490af85424eb7e7f3ae721cd67d.html
https://www.psy.lmu.de/exp/teaching/courses/w9-signal-detection.pdf
https://neuropsychology.github.io/psycho.R/2018/03/29/SDT.html
Demonstration https://elvers.us/perception/sdtCalculator/
Signal Detection Theory (SDT) attempts to address this shortcoming of classical psychophysics. Rather than having a single value, the threshold, SDT provides two parameters -- d' (dee prime) and β d' is similar to idea of a threshold. A person who has a low threshold (and therefore is sensitive to the stimulus) will have a largish d' while a person who has a high threshold (and therefore is insensitive to the stimulus) will have a d' that is close to 0. That is, d' is a measure of how sensitive the person is and is usually driven by bottom-up concerns.
β is a measure of how willing a person is to say that the stimulus was present. If you are expecting an important phone call, you are more willing to say that your phone is vibrating. That is, β measures top-down processing. A conservative person requires much evidence that the signal is present before they are willing to say that the stimulus is present. Such a person will have a β that is larger than 1. A liberal person requires little evidence that the signal is present before they are willing to say that the stimulus is present. Such a person will have a β greater than or equal to 0 but less than 1. When β equals 1, the person is unbiased -- neither conservative nor liberal.