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Single-Factor Experiments

Single-Factor Experiments. What is a true experiment? Between-subjects designs Within-subjects designs. What is a true experiment?. Experiment = study in which researcher has complete control over all aspects of the study 2 Essential features of experiments:

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Single-Factor Experiments

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  1. Single-Factor Experiments • What is a true experiment? • Between-subjects designs • Within-subjects designs

  2. What is a true experiment? Experiment = study in which researcher has complete control over all aspects of the study 2 Essential features of experiments: • 2 or more levels of an IV (control condition or group) • Random allocation of subjects to groups (if between-subjects)

  3. Some Terminology • IV = what the experimenter manipulates (varies) in an experiment; the hypothesized cause • DV = what the experimenter measures to test the hypothesis in an experiment; the hypothesized effect • Factor = IV • Level = condition = treatment: • One value of an IV • Control Variable – value held constant • Counterbalancing Variable • Confounded Variable – covaries with IV • Random variable – value is randomly varied

  4. Between-subjects designs • At least 2 conditions (groups) • Control and Experimental Condition, or • 2 or more levels of IV without “control” condition • Each subject is assigned to only one condition • Random assignment of subjects to conditions

  5. Between-subjects Designswith Multiple Conditions • More than 2 levels of the IV • Randomly assign subjects to conditions Design a between-subjects experiment with multiple levels to test the hypothesis: People work harder individually than they do in a group.

  6. Within-Subjects Designs • At least 2 conditions • Control and Experimental Condition, or • 2 or more levels of IV without “control” condition • Each subject is assigned to all conditions • Design a within-subjects experiment with multiple levels to test the hypothesis that people can perceive a light more quickly when their attention is focused where the light will appear. (Spatial Cuing)

  7. Order and Sequence Effects • Order effects • Result from the position in which a condition occurs (first, second, third, etc) • If the order of conditions is “A-B”, order effects on B would be the effects that result from B being in the second position in the list • Example: practice effects • Sequence effects • Result from which conditions precede or follow a condition • If the order of conditions is “A-B”, sequence effects on B would be the effects that result from B following A • Example: color perception

  8. Controlling Order and Sequence Effects • Controlling Order Effects • Counterbalance the position in which each condition appears • ABC, CBA, ACB: C occurs first, second, and third an equal number of times • Controlling Sequence Effects • Counterbalance what each condition follows • ABC, BAC : C follows A half the time, and follows B half the time.

  9. Counterbalancing in Within-Subjects Designs • Counterbalancing within subjects • Useful when each level of the IV occurs multiple times for each subject • Each subject gets all sequences of the conditions (ideally) • Counterbalancing within groups (not within each subject) • Useful when there are many (more than 2) conditions • Necessary if each condition occurs only once per subject • An equal number of subjects get each sequence of the conditions (ideally)

  10. Controlling Order and Sequence Effects Within Subjects • Only possible when each condition occurs at least twice for each subject • Method depends on how many times each condition occurs per subject: • Many times: randomize order of conditions • A few times: use block randomization of conditions • Twice: use reverse counterbalancing

  11. Controlling Order and Sequence Effects Within Groups • When it is not possible or practical to control within subjects • Necessary if each condition occurs only once for each subject • Sequences and orders controlled within a group of subjects, but not within each subject • Latin Square – a technique for partial counterbalancing (when full counterbalancing is impractical)

  12. Latin Square • A square matrix with length = number of conditions • Each condition occurs only once on each row and only once in each column: Subject #1: ABCD Subject #2: BCDA Subject #3: CDAB Subject #4: DABC • Controls order, but not sequence effects

  13. Balanced Latin Square • A Latin Square in which each condition is preceded by every other condition exactly once: Subject #1: ABCD Subject #2: BDAC Subject #3: CADB Subject #4: DCBA • Controls both order and sequence effects

  14. In-Class Group Assignment • Hypothesis: Meditation improves short-term memory for lists of words • Groups of 4 • Design an experiment to test this hypothesis • Define your IV and DV • State your prediction (hypothesized results) • Write a plan for the condition(s) each participant will be in, and in what order • Considerations: • Number of levels of IV? • Within or between-subjects design? • Random assignment to conditions? • Controlling order and sequence effects? • Other possible confounds?

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