ABSTRACT
A basic question when evaluating a manual handling task is: “How much should the worker be required to lift or lower or push or pull or carry?” Psychophysics is a particularly useful tool for answering that question. Psychophysics began over 165 years ago with the investigations of Ernst Heinrich Weber (1795-1878). Weber studied the sense of touch and found that a weight must be increased by a constant fraction of its value (about 1/40) for the perception of weight to be just noticeably different (JND), independent of the magnitude of the weight. He formulated Weber’s law:
delta I/I=constant
where I is the weight (intensity) and delta I is the increment that the weight must be increased to be just noticeably different (Snook, 1999).
