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Experimental Psychology PSY 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.
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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 A Reaction Time C Reaction Time C Minus A 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) C Reaction Time (total) B Minus C Baseline Identification Time Selection Time
The Subtractive Method • A: See a stimulus Press a key • B: See a stimulusIdentify which one it is Decide which key to press Press a key • C: See a stimulusIdentify which one it is Press 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.
