This part of ISO 20462 is part of a multiple-part standard pertaining to the subjective evaluation of pictorial still image quality. This part of ISO 20462 a) defines the units by which image quality is quantified (just noticeable differences, or JNDs); b) describes the influence of stimulus properties, observer characteristics, and task instructions on results obtained from rating experiments; c) provides a flow chart for choosing the preferred psychophysical method for determining image quality from among those defined in subsequent parts of ISO 20462