Experimental Designs: Between and Within Subjects Design: Chapter 8 & 9 Flashcards
between groups
- 2+ groups are formed at random from a
pool of subjects - each group receives a different experimental treatment (value of the IV)
- scores for the groups are
compared
between groups: obtaining results
- 1 score per individual
- these are independent measures
do scores vary within groups
yes
systematic variance
difference in the DV
looking for an effect of IV
to look for an effect of IV, compare mean scores (DV) for each group
determining statistical significance
compare between groups variance to within groups variance
F = between-group variance / within group variance
When there is a large within-group variance, it is
difficult to see an effect, we want to ____
minimize it
large versus small variance in groups
- large variance in between groups (BG) = good
- large variance in within groups (WG) = bad
keeping within group variance low
limit individual differences
- Standardizing procedures
- Holding a participant variable constant
- Increase sample size
individual differences in between-groups are usually ____
always a potential confounding variable for this design
making between groups as equal as possible
- created equally
- treated equally, except for IV
- composed of equivalent individuals
randomization
- participants randomly assigned to groups to
ensure groups are as equal as possible before
treatment or intervention - most powerful technique to control for the effect
of pre-existing differences
randomization vs random sampling
randomization:
- random assignment of Ss to experimental
or control groups in a particular study
random sampling:
- random selection of Ss from a larger
population to participate in a study
free random assignment
- groups are based on chance
- if 2 groups +, table of random numbers is used to guard against repetition
- should lead to equality, but no guarantee
randomization: matched group
- participants matched on critical variables that may act as important confounds
- inherent in within-groups designs.
- ex:
1. intelligence
2. gender
3. age
4. severity of illness
randomization: randomized blocks
groups of individuals are matched in blocks
matched groups
critical variables that can act as confounds
- intelligence
- gender
- age
- severity of illness
matching procedure
- Rank subjects on the variable for which
control is desired. May require pretest. - Segregate subjects into matched pairs
on that variable. - Randomly assign pair-members to the
conditions.
advantages of between-groups
- very simple design
- no carryover effects
disadvantages of between-groups
- requires many participants
- individual differences & environmental differences
- groups must be equivalent before the manipulation
within-groups design
- only one treatment group and each subject is
given all levels (or conditions) of the IV - comparison = between scores obtained at
different levels of the IV for same participants - each participant serves as their own control
categories of within-groups
- concurrent measures
- repeated measures
concurrent measures
- all levels of IV are present at the same time – choice paradigm
- subjects choose the value of the IV they prefer
Harlow: monkeys cloth mother
(comfort) versus wire mother
(food).
Measured time spent at
each mother.
This is an example of ______
concurrent measures