December 2010 // Volume 48 // Number 6 // Tools of the Trade // v48-6tt2
"I don't know" and Multiple Choice Analysis of Pre- and Post-Tests
Evaluation is an essential component of any Extension education program. One tool, the pre- and post-test, provides measurable evaluation data. Yet often the answer "I don't know" or all possible answers to a multiple choice question are not included in the repeated measure analysis. Because more than two answers are offered, the test of marginal homogeneity repeated measure analysis should be used, which takes into account all response sets when comparing a participant's pre- and post-test answers. This article provides an example of conducting and interpreting the test of marginal homogeneity.
Pre- and post- tests are a common method to determine if an Extension program has achieved its knowledge outcomes. When constructing a pre-and post-test, the format of the questions should be considered, especially because the question format will affect which repeated measure analysis should be conducted. If the pre-and post- tests contain more than two levels or categorical responses, for example "I don't know" or multiple choice answers, then the test of marginal homogeneity should be used. The test of marginal homogeneity extends the McNemar test to those cases where the variable of interest assumes more than two nominal or ordinal values for k χk table. The test of marginal homogeneity is an extremely important test yet an underutilized statistical method. The basic concept for the marginal homogeneity test is illustrated in Table 1 (Uebersax, 2006).
|1-Correct||2-False||3-"I don't know"|
|*P =Represents paired answers (e.g. answer 1 pretest and answer 1 postest = P11)
**p =Represents the total participants that provided a specific answer
Questionnaire Design: "I don't know"
It is common to ignore the "I don't know" answer. Just comparing the percentage of participants who correctly answer a question before and after a program does not include those who changed their answer to or from "I don't know." Therefore, the true the impact of an education program may be underestimated or overestimated.
Rockwell and Kohn introduced a post-then-pre test evaluation method (Rockwell and Kohn, 1989). This approach sought to reduce pretest bias due a participant's lack of prior exposed to a concept. They suggest that upon program completion, participants report both their prior and post knowledge/behavior simultaneously. However, participants may express what they feel the cooperative Extension educator wants to hear.
Offering "I don't know" as an answer is potentially a better means for addressing pretest bias. Education is defined as deliberately transmitting knowledge, skills, and values to another (Webster's New World Dictionary and Thesaurus, 1996). If a participant is unaware of an idea prior to the program, but acquires understanding; learning has occurred. This is the goal of Extension education.
Example: Calcium, It's Not Just Milk
The Nevada Calcium, It's Not Just Milk program will serve as an example of using the test of marginal homogeneity in Cooperative Extension. The Calcium, It's Not Just Milk program was initiated in 2000 and serves to increase the consumption calcium rich foods among middle school students. Educational methods have included direct instruction by the teacher or a guest dietitian, food tasting events, and poster displays. In 2006, 747 middle school students participated in the program. There were 8 students who lacked matching pre- and post-tests results or declining participation. The total sample size for analysis is 739.
Question 1 asked do "Young children need more calcium than teenagers?" It contained three possible answers: True (incorrect answer =1), False (correct =2), or I don't know=3. On the pre-test, 153 (20.7%) students answered the question correctly, and 406 (55%) answered the question correctly on the post-test.
Because this is a repeated measures question with three response levels in a one sample population, each pre-test response can be grouped with three of the possible post-test responses. Therefore there are nine response sets (Table 2).
|Young Children Need More Calcium than Teenagers?|
|Post-test Frequency (with percentage)||Total|
|Pre-test Frequency||True =1||False =2||I don't know = 3|
|True = 1||180 (24%)||219 (30%)||22 (3%)||421|
|False = 2||33 (4.5%)||115 (16%)||5 (1%)||153|
|I don't know =3||59 (8%)||72 (10%)||34 (5%)||165|
|Pre vs. Post Response Sets (9 response sets)|
|1:1||24% incorrect pre and post|
|1:2||30% incorrect pre than correct post|
|1:3||3% incorrect pre than I don't know post|
|2:1||16% correct pre and post|
|2:2||4% correct pre then incorrect post|
|2:3||1% correct pre then I don't know post|
|3:1||5% I don't know pre and post|
|3:2||8% I don't know pre then incorrect post|
|3:3||10% I don't know pre then correct post|
Computing the Test of Marginal Homogeneity in SAS®, requires three steps:
- Input your dataset in pre -and post-test format for each subject.
- Run SAS PRO CATMOD procedure.
- Interpret the SAS output
Collaborating with a statistician is recommended for database construction and conducting the SAS PRO CATMOD procedure.
Step 1: Input Dataset
Data can be entered into SAS in a variety of ways. You can directly type your research data into SAS, but this requires some finagling column attributes. It is easier to create a Window Excel file (Table 3). If your research data is spread out into multiple Windows Excel spreadsheets, merge them before importing your data into SAS. SAS can actually merge files, but it is easier to conduct this in Excel. Using the SAS import wizard will assist in importing your data.
|SUBJID||gender||age||Question 1 Pre-test (q1pre)||Question 1 Post-test (q1post)|
Step 2: SAS Procedure
An example of the procedure is:
WEIGHT SUBJID;title2 'Test of Marginal Homogeneity';
repeated time 2;
Step 3: Interpret SAS Output
The SAS output will contain several sections of information (Table 4). The Analysis of Variance provides the degrees of freedom, chi-square value for intercept and time, and statistical value of Percent Chi Square. Because time was <0.0001, the test of marginal homogeneity is interpreted as having no marginal homogeneity between pre- and post- test. In other words, the distribution of answers pre-test differs significantly from the answers from the post-test in the same subjects.
SAS Test of Marginal Homogeneity Output from Calcium, It's Not Just Milk Result for Question 1 "Young Children Need More Calcium Than Teenagers?" (n=739)
Questionnaire Design: Multiple Choice
Multiple choice questions with one correct answer should also be an evaluated by the test of marginal homogeneity. The process and result interpretation are similar to what is conducted when "I don't know" is provided as an option to a question.
An example multiple choice questions with one correct answer from the Calcium, It's Not Just Milk program is Question 8: "What is the serving size of this item on this food label?" Possible answers were 1=1 cup (correct), 2=1/2 cup, 3=1 ½ cups, 4=2 cups. The test of marginal homogeneity can be used with ordinal data (Table 5).
The SAS Output is very similar, except there are 16 response levels.
|What is the Serving Size of this Item on this Food Label?|
|Post-Test Frequency (with percentage)||Total|
|Pre-test Frequency||1 Cup= 1||½ Cup = 2||1 ½ Cups =3||2 Cups = 4|
|1 Cup= 1||12
|½ Cup = 2||499
|1 ½ Cups =3||46
|2 Cups = 4||23
|*Frequency missing =16|
|Pre vs. Post Response Sets (16 response sets)|
|1:1||1.6% correct pre and correct post|
|1:2||1.4% correct pre than incorrect post|
|1:3||1.1% correct pre than incorrect post|
|1:4||0.5% correct pre than incorrect post|
|2:1||68.3% incorrect pre then correct post|
|2:2||2.1% incorrect pre then incorrect post|
|2:3||4.7% incorrect pre then incorrect post|
|2:4||2.6% incorrect pre then incorrect post|
|3:1||6.3% incorrect pre then correct post|
|3:2||1.1% incorrect pre then incorrect post|
|3:3||2.6% incorrect pre then incorrect post|
|3:4||1.0% incorrect pre then incorrect post|
|4:1||3.2% incorrect pre then correct post|
|4:2||0.5% incorrect pre then incorrect post|
|4:3||1.8% incorrect pre then incorrect post|
|4:4||1.4% incorrect pre then incorrect post|
|*total percentage >100 due to rounding|
SAS Test of Marginal Homogeneity Output From Calcium, It's Not Just Milk Results for Question 8: "What is the Serving Size of this Item on this Food Label?" (n=731)
A statistical significant increase was found answering Question 8 between pre- and-post tests. Only 4.7% (34/731) of participants answered the question correctly pre-test and correctly post-test, while 79.3% (580/731) answered the question incorrectly pre-test and correctly post-test (p<0.001).
The evaluation component is an important aspect in developing an education program. When a pre- and post-test is selected, the number of possible answers determines the statistical test that will be conducted. A statistician should be consulted early in project creation. Extension educators cannot ignore the" I don't know" or multiple choice questions when more than two levels are provided in the questionnaire. The test of marginal homogeneity is a statistical analytic test that will provide a more appropriate evaluation. This reinforces the need to begin with the-end-in-mind in program evaluation.
Agnes, M. (1996). Webster's new world dictionary and thesaurus, New York, NY: Simon & Schuster, Inc,
Rockwell, S. K., & Kohn, H. (1989). Post-Then-Pre Evaluation. Journal of Extension [On-line], 27(2) Article 2FEA5. Available at: http://www.joe.org/joe/1989summer/a5.php
Uebersax, J. (2006). McNemar test of marginal homogeneity. [On-line], Retrieved December 8, 2009 from: ht,tp://john-uebersax.com/stat/mcnemar.htm