Weber’s Law in AP Psychology

The ratio that sets the just-noticeable difference

The just-noticeable difference (JND), also called the difference threshold, is the smallest change in a stimulus a person can detect about half the time. Weber's law is the rule that predicts how big that JND will be. The JND equals the starting intensity multiplied by a fixed fraction, often called the Weber fraction. The principle is named for Ernst Weber, a German physiologist in the 1800s who studied judgments of lifted weights.

Here's a worked example with simple numbers. Say someone can just barely tell that a 20-ounce bag got heavier when 1 ounce is added. Their Weber fraction is 1/20, or 5%. For a 60-ounce bag, they'd need about 3 extra ounces (5% of 60). For a 200-ounce bag, they'd need about 10.

The absolute change grows, but the ratio stays the same. The fraction itself varies with the sense, the specific task, and the conditions, and the law works best in the middle range of intensities, so treat it as a strong approximation rather than an exact rule.

Weber's law vs. absolute threshold vs. sensory adaptation

These three show up together in Topic 1.6, and AP questions may ask you to distinguish them. The fastest way to sort them is to ask what the person in the scenario is doing.

• Absolute threshold: detecting a stimulus at all, going from nothing to something (the faintest sound noticed at least 50% of the time).
• Weber's law and the JND: noticing a change in a stimulus that's already present, where the change needed depends on the starting amount.
• Sensory adaptation: a stimulus stays constant and sensitivity to it drops over time, like no longer smelling your own kitchen.

The JND and Weber's law also get mixed up with each other. The JND is the measurement (the smallest detectable change, such as 3 ounces). Weber's law is the pattern that measurement follows (in the weight example, the detectable change is about 5% of the starting weight, whatever that weight is). If a question asks for the name of the minimum detectable change, the answer is the JND. If it asks why a bigger change was needed when the original stimulus was stronger, the answer is Weber's law.

How relative change shapes what people notice and do

A 1-ounce change is just noticeable on a 20-ounce bag, but a 200-ounce bag needs about 10 extra ounces. Your senses track relative change, so whether a change gets noticed, and acted on, depends on the starting point. That's how Weber's law fits Unit 1: Biological Bases of Behavior, Topic 1.6 Sensation, which asks how the process of sensation connects to behavior and mental processes.

That shows up in everyday behavior. Turning on one lamp is obvious in a dark room and barely registers in a room already full of light. When music is playing softly, a small turn of the volume knob is easy to notice, but when the music is already loud, the volume has to go up by a much larger amount before anyone notices the change. For sensory stimuli, a change of the same physical size may be easier to detect at a lower starting intensity.

Spotting Weber's law in a scenario or study summary

Weber's law can be tested through application questions, including standalone or set-based multiple-choice questions. These describe someone who notices, or misses, a change and ask which sensory principle explains it. The cue to look for is that the result depends on how intense the original stimulus was. If the scenario is about detecting something faint for the first time, or about a steady stimulus fading, it's pointing to absolute threshold or sensory adaptation instead.

The principle also fits the research skills in the free-response section. In an Article Analysis question about a perception study, you might identify the independent variable as the starting intensity (for example, the weight participants began with) and the dependent variable as the smallest change they reported noticing. If you use it as course knowledge in an Evidence-Based question, a strong sentence names the principle and ties it to the specific numbers or situation: "Because the original weight was heavier, a larger added weight was needed before participants detected the difference, which is consistent with Weber's law."

The common wrong prediction is assuming a person always needs the same fixed amount of change, like "1 pound is always noticeable." On a calculation-style item, find the fraction from the first pair of numbers, then apply that same fraction to the new starting value.