Lecture 10 Psyc 300A

Types of Experiments Between-Subjects (or Between- Participants) Design –Different subjects are assigned to each level of the IV –Random assignment to conditions –Difference between random assignment and random sampling Within-Subjects (or Within-Participants, or Repeated Measures) Design –Same subjects in all levels of the IV

True vs Quasi-Experiments True experiments have IVs that can truly be manipulated Quasi experiments include quasi-experimental IVs that cannot be manipulated; uses naturally-occurring groups of participants (participant variables) –Participants cannot be randomly assigned to groups –Looks like an experiment, but can’t get at causation

Practice: Types of Experiments ITV study Pepsi Challenge experiment from HW4 A researcher is interested in whether listening to classical music improves spatial ability. She randomly assigns participants to either a classical music condition or a no-music condition. Participants serve in the music or no-music conditions, then are tested on their spatial ability.

Review: Properties of Studies Internal Validity –Extent to which observed relationships in a study (scores) reflect relationships between hypothetical variables. External Validity –Extent to which the results of a study can generalize to other people and settings outside the study

Threats to Internal Validity These are threats because each represents potential confounding Nonequivalent control group/Selection: Occurs when one group initially differs from another History: Events take place between measurements that are not related to IV Maturation: Participants change over time Testing: Effects of repeated measurement on same person

Threats to Internal Validity Regression to the mean: Arises when participants are selected based on extreme scores Instrumentation: Changes may occur in the measurement device over time (including human measuring devices) Mortality or attrition: Occurs when participants drop out differentially Diffusion of treatment: Occurs when participants tell others about a study

Practice: Threats to Validity Psychologists investigated the relation between playing action video games and the ability to monitor the visual environment. They recruited a group of male undergraduates who played action video games at least one hour a day, four days a week over the previous six months. A control group of non-video game players also participated. The researchers discovered that video game players were better at monitoring visual elements unrelated to the video game while playing. The researcher concluded that playing video games increases one’s capacity to pay attention to details in a visual environment.

Practice: Threats to Internal Validity You read in a health magazine about a study in which a new therapy technique for depression was examined. A group of depressed volunteers participated in the study lasting 9 months. There were 50 participants at the beginning of the study, all of whom received the new therapy technique, but only 29 completed. The researcher claimed that of those who completed the program, 85% improved.

Experimenter and Participant Effects Participant Effects –Participants are not passive –Participant reactivity: behavior changes when participants know they are watched (may respond with cooperation, antagonism, social desirability) –Respond to demand characteristics

Social Desirability The pressure that participants feel to respond as they think they should, not as they actually feel or believe. The acceptable or PC response

Demand Characteristics These are cues that come from the experimenter or the experimental situation that tell a participant about the purpose of the experiment Participants may be right or wrong in their guesses Participants may change their behavior (e.g., become more or less cooperative)

Experimenter Effects Experimenter bias occurs when the behavior of the experimenter in some way affects the results of the study –Experimenter expectancy effects –Experimenter attributes

Experimenter Expectancy Effects When experimenter’s knowledge or belief about participants cause participants to act different from normal May involve –Demand characteristics (leaking) –Interpretation of responses –Subtle, not purposeful bias

Experimenter Attributes When characteristics of experimenter affect participants Examples: Age, ethnicity, attractiveness, gender, extraversion, controlling May limit external validity

Controlling Participant and Experimenter Effects Deception Blind studies Automation

Threats to External Validity Generalizing to populations Generalizing from lab settings Review: Replication –Literal (exact) –Conceptual