Experimental Control

I.  Introduction:  Goals

II.  Types of Experimental Control

III.  Means of Achieving Control

IV. Summary

I.  Introduction:  Goals

A.  Infer true causal relations

  (threats to validity)

  constant influence of extraneous factors

B.  Determine true functional relations

 2.  Hold all else constant

Example:   Memory as a function of  retention interval

Jenkins & Dallenback (1924)

Two subjects studied non-sense syllables
They were tested either immediately, 1, 2, 4 or 8 hours later
During the retention interval they either were awake doing daily activities, or they slept.

Classic Shape of the curve: (Woodworth, 1938)

Rubin, Hinton, & Wenzel (1999)

    Used a continuous recall task to get a lot of data to precisely model the time course of retention.

    The best fitting curve was the sum of three different log functions.

II.  Types of Experimental Control

A.  Experimental versus Control groups

Experimental group:  treatment
Control group: no treatment

Example:  use of a computer to aid instruction

B.  Control over the levels or values of the independent variable

Example:  the effects of cigarette smoking on the number of colds a person gets in a year
 - must control cigarette smoking.

C.  Control over the experimental setting

Example:  controlling the noise in an auditory perception experiment

D.  Control Variable:

Example:  survey of attitudes toward sexual harassment in the workplace

Control:  gender, age, race, etc.

III.  Means of Achieving Control

A. Randomization

1.  Random selection

- issue external validity of the results
- not really possible
 

2.  Random Assignment:

- easy to accomplish
- necessary to protect internal validity

A.  Randomization (cont)
 

3.  What randomization accomplishes

 b) controls both known and unknown variables.
 

4.  Things to randomize:

•  - subject assignment to condition
•  - use of research materials or items  (e.g., words in a list)
•  - order of treatments  (e.g., drug, no drug, . . .)
•  - order of questions on a questionnaire

5.  How to randomize

• - step 1:  assign values to outcomes  (e.g., heads = females)
• - step 2:  create a random series of outcomes

 - flip coins
 - draw cards

 - use a random numbers table

Random numbers table (p. 446 )

B.  Counterbalancing

Insures that each condition precedes and follows each other condition an equal number of times.
Counterbalancing (cont)

1) Things to counterbalance:

With 2 levels of an independent variable:

2) Intra-subject counterbalancing:

A B B A

Without counterbalancing:  Caffeine, No Caffeine

With counterbalancing:
No Caffeine, Caffeine, Caffeine, No Caffeine

3) Intragroup Counterbalancing:

Example        Group 1: Caffeine, No Caffeine
                     Group 2:  No Caffeine, Caffeine

With four levels of an independent variable:

 A= caffeine
 B= cold medicine
 C= alcohol
 D= no medication

Latin Square
A B C D
B C D A
C D A B
D A B C
 

4)  Counter balancing a subject variable:

Rank the subjects:  1, 2, 3, 4, 5, 6, . . .
Cond 1:  1, 4, 5, 8
Cond 2:  2, 3, 6 ,7
 

C.  Matching

Things to match:

•  subject variables: IQ, SES, Age, Gender, . . .
•  stimulus variables:  word frequency, word length . . .
•  environmental variables:  time of day, room . . .

1) Hold the variable constant:

 problem:  limits external validity

2.  Matched Groups:

Example:  construct 2 groups that have the same mean reading level

Problem: may hold the mean constant, but the range and variance may be very different.
 

3.  Incorporate the Variable as an additional independent variable.

Problem:  may lead to very large experimental designs.

4.  Yoking

Brady (1958) "executive monkey"
(an example in yoking)

• Monkeys trained to avoid an electric shock by pressing a lever whenever a light came on.
• Each press of the lever postponed the shock for 20 sec.
• Four pairs of monkeys were paired so that one monkey received the same shock as the ìexecutiveî that could avoid the shock.

And now for the rest of the story!

Executive Monkeys Part Deux
 

• How were the monkeys select for the executive positions?
•  Monkeys that were easy to train (high responders) were selected as executives.  Weiss (1971) demonstrated that high responders were more prone to ulcers than low responders.
• Study repeated with the animals equated for activity level.
• Animals that had control had less severe ulcers than those that do not have control

(Faltz & Millett, 1964; Naletson, 1976)

IV. Summary

A.  Goal: to conduct research that is free from threats to validity.

B.  This means exercising control over the important variables

C.  Primary Means of Achieving that control:

•  - randomization
•  - counterbalancing
•  - matching