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Weber's Law is related to the Just Noticeable Difference (also known as the difference threshold), which is the minimum difference in stimulation that a person can detect 50 percent of the time. But Ernst Weber noted that for people to really perceive a difference, the stimuli must differ by a constant "proportion" not a constant "amount".
For example, if you are buying a new computer that costs $1,000 and you want to add more memory that increases the and the price $200 (a 20% increase), you might consider this too much additional money to spend. However, if you were buying a $300,000 house a $200 feature may seem like nothing. It might take an additional $10,000 to make you stop and think if it's too much to spend. In this example, the amount stays the same ($200), but the proportion changes and that's what makes the perceptual difference.