MEASURING PRECISION AND ACCURACY

Method of Limits

Method of Constant Stimuli

Method of Adjustment

PRECISION OR ACCURACY

Precision & Weber's Law

(In)accuracy & M-L Illusion

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OVERVIEW AND OBJECTIVES

According to Weber's Law, the just noticeable difference (JND) or variable error is proportional to the size of the standard stimulus. Mathematically this can be written as:

JND = K x S

where S is the size of the standard and K is a constant called the Weber fraction.

K is always less than one and represents the proportion by which the standard stimulus needs to be increased or decreased before a reliable discrimination can be made. A small Weber fraction indicates high precision.

If Weber's Law holds true, a straight line will fit the data. In addition, when the standard stimulus length is zero, the JND will also be zero.

This section will show you:

1. How to calculate the Weber fraction
2. How precision and variable error vary with stimulus size;
3. That accuracy and constant error do not vary with stimulus size; and
4. The mathematical form of Weber's Law.