# INTRODUCTION

## REQUIREMENTS

- You will need a pencil, calculator and a worksheet to take notes and record results.
- A calculator is available in this program but you may want to use your own.

This package shows how to measure precision and accuracy using three classical psychophysical methods and demonstrates how to describe experimental results in terms of these two concepts.

The two concepts are closely related to concepts you encounter in other psychology courses. Precision is related to the concept of variability (standard deviation, quartile deviation, or range) discussed in a statistics course, and to the concept of reliability or random error ("noise") discussed in a course in measurement. Accuracy is closely related to a central tendency (mean, median, or mode) discussed in a statistics course, and to the concept of validity or "bias" discussed in a course in measurement.

One psychophysical question concerns the limits of discrimination: how different must two stimuli be for the difference to be detectable? In the psychophysical literature the smallest difference in stimulation which is reliably discriminated from a standard (fixed) stimulus is called the just noticeable difference (JND). The set of stimuli which are not noticeably different from the standard stimulus defines the interval of uncertainty (IU). When the just noticeable differences (JND) or the interval of uncertainty (IU) is small, the set of adjustments (or judgements) is said to be high in precision.

Another question concerns what two stimuli appear to be equal. The average stimulus size which appears to equal the size of a standard stimulus is known as the point of subjective equality (PSE). When the difference between a PSE and a point of objective equality (POE) is small, the set of adjustments (or judgments) is said to be high in accuracy.

The three classical psychophysical methods to measure precision and accuracy are: the Method of Constant Stimuli, the Method of Limits, and the Method of Adjustment. Each method has unique features which differentiates it from the other methods. The differences among these methods are discussed in the Postscript section of the manual.

** By working with the three psychophysical methods you will: **

- Gain direct, hands-on experience in the area of psychophysics
- Learn the differences among the three methods
- Learn how to analyze data for each method and
- Learn a number of concepts in psychophysics.

Weber's Law and the Mueller-Lyer Illusion in this package demonstrate that precision and accuracy are distinct concepts. In Weber's Law, precision varies as a function of standard size, but accuracy is relatively unchanged. Conversely, in the Mueller-Lyer Illusion, accuracy is affected by the direction of the arrows while precision is not.

**By working with the two concepts you will:**

- Understand the relationship among precision,variable error, and the just noticeable difference (JND)
- Understand the relationships among accuracy, constant error, point of subjective equality (PSE), and point of objective equality (POE)
- Gain an understanding of Weber's Law by collecting and analyzing your own data and
- Gain experience in measuring the extent of illusions.