Experimental PsychologyPSY 433

Ch. 8, pg 207-209

Reaction Time as a Dependent Variable

Some Terminology

Dependent variable (DV) – the variable measured and recorded by the experimenter.

Independent variable (IV) – the variable manipulated by the experimenter. In an experiment, changing the IV should

produce a corresponding change in the DV, demonstrating a causal relationship.

Studies can have multiple DV’s (called multivariate) or multiple IV’s (called factorial).

Donders Tasks

S1 R1 Donders A

S1 R1 Donders BS2 R2

S1 R1 Donders CS2

Measuring RT – Donders A

The two most common DVs in experimental psychology are percent correct and reaction time (RT).

There are several types of RT Reaction time vs response time

Simple RT (Donders A): the time between the onset of a stimulus and the onset of a response.

Requires time for sensory processing, nerve conduction, and responding.

Measuring RT – Donders C

Go NoGo (Donders C): the time between the onset of one of multiple possible stimuli and the onset of a response to only a single one of those stimuli.

Requires time for all processes in simple RT plus time for stimulus identification.

Measuring RT – Donders B

Choice RT (Donders B): the time between the onset of one of multiple possible stimuli and the onset of a unique response to each stimulus.

Requires time for all processes in simple RT plus time for stimulus identification and time for response selection.

Donders A -- Simple

A Reaction Time (total)

Perceive stimulus and press key

Donders C – Go-No Go

C Reaction Time (total)

Perceive stimulus, decide if it is the one requiring you to press a key, press the key

Isolating and Measuring the Two Components of the Task

C Reaction Time

C Minus A

A Reaction Time

The two steps (perceive a stimulus and decide whether to press a key or not) can be separated and the time needed for each determined by subtraction.

Donders B -- Choice

B Reaction Time (total)

B Minus C

C Reaction Time (total)

Baseline Identification Time

Selection Time

The Subtractive Method

A: See a stimulusPress a key

B: See a stimulusIdentify which one it isDecide which key to pressPress a key

C: See a stimulusIdentify which one it isPress a key or not

Measuring the Separate Parts

A is the Simple task B is the Choice task C is the Go-NoGo task A is how long it takes to see a stimulus and

press a key. To figure out how long it takes to identify the

stimulus, subtract A from C. To figure out how long it takes to decide

which key to press, subtract B from C.

Subtractive Method in Experiments

Define two or more groups (levels of the IV): Group 1 – Control group Group 2 – Experimental (treatment) group

Measure both groups Subtract Group 2 from Group 1

The difference is the effect The effect is the amount attributable to

whatever happened differently in Group 2. If there is no measurable difference between

the two, there is no effect.

Reaction Time Expt (Uncleaned)

TaskSimple Mean P 1 1.042

StDevP 1 1.975Min P 1 0.253Max P 1 9.22N P 1 20SE P 1 0.44 P 1

Go/NoGo Mean P 2 0.426StDevP 2 0.067Min P 2 0.317Max P 2 0.53N P 2 20SE P 2 0.02 P 2

Choice Mean P 3 0.477StDevP 3 0.100Min P 3 0.301Max P 3 0.71N P 3 20SE P 3 0.02

Reaction Time Expt (Cleaned)

TaskSimple Mean P 1 0.343

StDevP 1 0.072Min P 1 0.253Max P 1 0.50N P 1 19SE P 1 0.02 P 1

Go/NoGo Mean P 2 0.426StDevP 2 0.069Min P 2 0.317Max P 2 0.53N P 2 19SE P 2 0.02 P 2

Choice Mean P 3 0.454StDevP 3 0.077Min P 3 0.301Max P 3 0.64N P 3 19SE P 3 0.02