Download Welcome to Psyc 300A: Understanding Psychological Research I

Lecture 10
Psyc 300A
Types of Experiments
• Between-Subjects (or BetweenParticipants) 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