Final answer:
The just-noticeable difference (JND) or difference threshold is the minimum weight difference a person can detect 50% of the time. For a one-pound bag of rice, this may be 0.2 pounds (20%), and by Weber's Law, this would also be 20% for a 20-pound bag, equating to 4 pounds.
Step-by-step explanation:
The just-noticeable difference (JND), also known as the difference threshold, refers to the minimal variation in stimuli that an individual can detect 50% of the time. In the context of weight, it's the smallest detectable difference between two weights that a person can sensibly perceive. When considering a one-pound bag of rice versus a larger bag, the JND might be for instance, 0.2 pounds which represents a 20% increase. According to Weber's Law, the JND is a constant proportion of the original stimulus rather than a constant amount. Therefore, for a 20-pound bag, if the law holds true, the JND would also be 20% or 4 pounds. Testing the JND requires incrementally increasing weights, which allows for the determination of the point at which the difference is detectable.
Hypotheses about the JND in terms of percentage of the overall weight can be developed and tested using the scientific method. For example, if it's hypothesized that the JND percentage is consistent regardless of the magnitude of the weight, this would mean that both small and large increments of weight must exhibit JNDs that are proportionate to their respective base weights. This can be explored by comparing noticeable differences at different weights and observing if the detection pattern conforms to a percentile increment.