TYPES OF EXPERIMENTAL DESIGN

 TYPES OF EXPERIMENTAL DESIGN 

Three types of experimental designs 

A. BETWEEN-SUBJECTS DESIGN: - Different groups of subjects are randomly assigned to the levels of the independent variable.

B. WITHIN-SUBJECTS DESIGNS: - Only one group of subjects; subjects receive all levels of the independent variable at different times.

C. SINGLE-SUBJECT DESIGNS: - Use the same method of varying the level of the independent variable used by the with-in subjects designs. - Focus on change of individual subjects under the different treatment conditions.

WITHIN-SUBJECTS DESIGNS (repeated measures) 

The comparison of treatment effects involves looking at changes in performance within each subject across treatments. - Expose a single group of subject to all the treatments

TYPES OF WITHIN-SUBJECTS DESIGNS (family of designs)

1. SINGLE-FACTOR TWO-LEVEL DESIGN: 2 levels of a single independent variable. All subjects receive both levels of the variable. - If dependent variable not strongly affect by subject- related variable design will be less effective.

2. SINGLE-FACTOR MULTILEVEL DESIGNS: - More than two levels of the independent variable - Single group of subject is exposed to three or more levels of a single independent variable.

3. MULTIFACTOR WITHIN-SUBJECTS DESIGNS: Includes two or more independent variables

Factorial Designs: Each subject is exposed to every combination of levels of all the factors (independent variable) - Have main effects and interactions.

4. OTHER WITHIN-SUBJECTS DESIGNS: Nonfactorial design

5. MULTIVARIATE WITHIN-SUBJECTS DESIGNS: Use of more than one dependent variable

WHEN TO USE A WITHIN-SUBJECTS DESIGN:

1. SUBJECT VARIABLES CORRELATED WITH THE DEPENDENT VARIABLE: - Use when subject differences contribute heavily to variation in the dependent variable.

2. ECONOMIZING ON SUBJECTS: - Use when # of subjects is limited and carryover absent or minimized.

3. ASSESSING THE EFFECTS OF INCREASED EXPOSURE ON BEHAVIOR: - Measured a number of trials, passage of time, etc. - Looking at changes as a function of earlier exposure

A. ADVANTAGES: - Close to matched groups. Within provides the ultimate in matching of characteristics. - Tends to be more powerful than equivalent between-subject design. - Increase power may allow for use of less subjects. 

B. DISADVANTAGES: - Amount of time in the experiment - Carryover effects

C. SOURCES OF CARRYOVER EFFECTS: Potential sources for carryover effects 

1. Learning: if learn a task; second performance is likely to be better if similar.

2. Fatigue: may lead to deterioration in later performance.

3. Habituation: reduction in responsiveness for repeated exposure.

4. Sensitization: exposure to one stimulus can cause subjects to respond more strongly to another stimulus.

5. Contrast: exposure to one condition may alter responses in other conditions

6. Adaptation (adjustments): may lead to different results due to adaptive changes (e.g., drug tolerance) 

D. DEALING WITH CARRYOVER EFFECTS: Three ways to deal with carryover effects. 

1. COUNTERBALANCING: assign various treatments of the experiment in a different order for different subjects.

a) COMPLETE COUNTERBALANCING: every possible ordering of treatments assign at least 1 subject to each ordering.

1) Every treatment follows every other treatment equally often.

2) Every treatment appears equally often in each position. - Minimum number of subjects is equal to the number of different ordering of the treatments. - k treatments have k! (k factorial). 3 x 2 x 1 = 6;

For four treatments: 4 x 3 x 2 x 1 = 24

b) PARTIAL COUNTERBALANCING: include only some possible treatment orders. Orders retained are chosen randomly from total set. Latin Square Design: partially-counterbalanced design - Number of treatment order equivalent to number of treatments. - Each item appears exactly once in each column and row.

2. TAKING STEPS TO MINIMIZE CARRYOVER: if minimize the error variance – increases power - Pre-train subject - Allow breaks

3. MAKING TREATMENT ORDER AN INDEPENDENT VARIABLE: Order of treatment as a second independent variable.

BETWEEN-SUBJECTS DESIGNS (Family of Designs)

SINGLE-FACTOR RANDOMIZED GROUPS DESIGNS

Randomized Group Design: randomly assign subjects to level of independent; variable to form “groups” of subjects

1) THE RANDOMIZED TWO-GROUP DESIGN: 

+ Randomly assign subjects to two groups + Expose each to different levels of the independent variable. 

ADVANTAGES: simple to carry out 

1. Economical in terms of time and material. 

2. No Pre-testing of subjects in necessary 

DISADVANTAGES: 

1. Provides limited amount of information about effects of independent variable. 

2. Sensitivity:

2) THE RANDOMIZED MULTIGROUP DESIGN 

1. Single-Factor Parametric Design: assess independent variable at more than two levels.

2. Single-Factor Nonparametric Design: Nonparametric Design - nominal scale

3. Multiple Control Group Design: - Include multiple control groups when a single control group is not adequate to rule out alternative explanations of your results.

Small-N designs

Single Subject Research Design (SSRD) involves studying a single individual or system by taking repeated measurements of 1 or more dependent variables and systematically applying & sometimes, withdrawing or varying the independent variable.

Pseudo-experiment

It is a research design in which someone tests a claim about a variable (e.g., a product, a charm, a clinical treatment) by exposing people to the variable of interest and noting that these people think, feel, or behave as expected. People often treat these as if they were true experiments.