An investigator designs an experiment to see the effect of illumination level and duration of visual signal on detectability of change in illumination in a dark room on 120 randomly selected college-going male students. She decided to use low and high levels of illumination and three levels of duration of stimulus — 30, 50 and 70 milliseconds — through an electric chronoscopic instrument. The investigator used a separate-group design for the various combinations of treatments. Participants were required to press a 'Yes' button if they detected the signal or a 'No' button if they did not. Each participant was given a total of 50 trials within which 10 signals were randomly presented. Proportions of hits and false alarms were computed for each participant and analysed to answer the research questions.