Random Assignment to Control Participant Variability

For this assignment, we will be using the technique of the "flipped classroom". You will be asked to watch the video posted in Moodle of me explaining the purpose and method of randomly assigning participants to conditions as a method for controlling individual differences in an experiment.

Please ask any questions you have about the video content and I will answer them before class. We will complete this assignment together in class on the day the assignment is due. You will need to be prepared for the class by having watched the video, asking questions you might have about the content and printing the assignment so you can complete it in class (you can complete the assignment in class on the computer if you wish.

Person #
Condition
Age
IQ
Height
Anxiety
Self-esteem
1
C E
16
145
73
10
42
2
C E
17
73
64
12
48
3
C E
21
86
70
9
30
4
C E
39
108
71
1
36
5
C E
18
58
72
5
38
6
C E
37
109
70
9
42
7
C E
31
91
68
7
28
8
C E
25
124
73
9
39
9
C E
23
64
66
6
40
10
C E
21
121
61
6
36
11
C E
28
93
71
5
45
12
C E
21
107
74
6
44
13
C E
26
94
64
4
43
14
C E
25
114
72
9
35
15
C E
31
78
68
7
40
16
C E
15
104
71
6
45
17
C E
25
128
68
7
42
18
C E
17
81
66
7
41
19
C E
21
136
65
9
38
20
C E
23
97
75
6
42

Experimental Group (n = ______) Control Group (n = ________)

#
Age
IQ
Height
Anxiety
Self Esteem
#
Age
IQ
Height
Anxiety
Self Esteem
Me Mc
Purpose

This homework exercise is designed to give you practice randomly assigning people to conditions. It will also encourage you to think about the efficacy of random assignment for making sure that subject variables are spread out evenly across conditions.

Procedure

- Randomly assign the people in the first table to either an experimental or a control group using the random number table in your textbook. Starting at column 1-5, row 1 and moving down (row 2, row 3, row 4....), assign even numbers to the experimental group and odd numbers to the control group.

- In first table above, circle either the C (control group) or the E (experimental group) next to the person's number to indicate the group to which the person was assigned.

- When you have assigned the 20 people to their conditions, record their characteristics in the text table.

For example, if person #1 is assigned to the Experimental group, you would enter their number in the top, left space of the table. You would then enter Age (16), IQ (145), Height (73), Anxiety (10), and Self Esteem (42) in the appropriate cells of the table.

The first person assigned to the control condition would be entered on the same row, on the right side of the table.

Remember: If you have filled up all the rows in one condition, then the rest of the participants are automatically assigned to the other condition

- Compute the group means for each of the variables listed and enter them (rounded to the nearest hundredth of a point) in the spaces provided at the bottom of the table.

Me = Group means for experimental group

Mc = Group means for control group

Questions

  1. On the whole, are the two groups as similar as you expected? (This question could be answered correctly with either a yes or a no. The important thing is to explain your answer.)

  2. Remembering (a) that any differences you observed must have been the result of chance and (b) that experimenters typically take it for granted that their experimental and control groups are identical prior to any manipulations, imagine that your participants differed noticeably on only one variable - age.

    • Describe briefly an experiment or kind of experiment in which this preexisting difference between your groups might have disastrous consequences.

    • Describe a different kind of experiment in which this particular difference would be unlikely to pose any problems. Be specific about why the failure of random assignment would be problematic in one study and would not be problematic in the other.

  3. Next, assume that you have a group of 40 people at your disposal rather than 20.

    • What advantages and disadvantages would the extra 20 people pose? Make sure you frame at least one of your answers to this question in terms of the goal/purpose of random assignment.

Creative Commons License
This work by Cynthia Sifonis is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License.