PSYC 201 -- T-test lab
An investigator does an experiment to test the idea that caffeine has an adverse effect on memory
function. She assigns 10 participants at random to a group that will get the equivalent of four
cups of coffee before they take a test of memory function. She assigns 10 additional participants
to a group that gets a placebo before taking the same test of memory function. In the test of
memory function, participants are given a list of twenty words to remember and then asked to
recall as many words from the list as they can. The researcher predicts that participants in the
caffeine condition will remember significantly fewer words than participants in the placebo
condition.
The data for the participants in the two groups are presented below.
Caffeine Group
------------------8
10
7
9
11
9
8
10
9
11
Placebo Group
------------------12
9
14
10
11
13
14
10
9
11
Please address each of the following questions.
a.
b.
c.
d.
e.
Please write both the null and alternative hypotheses.
Please write the decision rule for rejecting the null hypothesis.
Using SPSS, please determine the observed value for t.
State your decision as to whether to reject the null hypothesis or not.
Please write a conclusion sentence for this test in APA format.
Below are instructions for using SPSS to get the observed value for t…
Entering the data into the SPSS spreadsheet
There are two things that you know about each participant. First, you know which level of the
independent variable they’ve been assigned to. In other words, each participant is in the caffeine
condition (group 1) or the placebo condition (group 1). That means for a variable that we might
name caffgrp each persons will get a score of “1” if they’re in the caffeine group and a score of
“2” if they’re in the placebo group. The variable name for this column would be caffgrp. In this
example, there will be 20 rows in the spreadsheet because there are 20 participants. The only
other thing we know about each participant is their score on the dependent variable, memory
function. These scores go in a second column that is labeled with the variable name memory.
So that it. The data set has 20 rows (for the 20 participants) and it has two columns of numbers
(for the two variables).
Independent-samples t-test using SPSS

Click Analyze

Click on the Compare Means pull-down menu

Click on Independent-Samples T Test. You should now see the Independent-Samples T
Test window.

Move the variable memory over to the Test Variable(s) box. The “test variable” is the
dependent variable.

Move the variable caffgrp over to the Grouping Variable box. The “grouping variable” is the
independent variable.

Click on the Define Groups button. There is where you tell SPSS which groups you want to
compare. Enter a “1” for the Group 1 box and a “2” for the Group 2 box. Then click
Continue.

Click OK back at the Independent-Samples T Test window.
Using the SPSS output window for an Independent-Samples T Test

In the Group Statistics box you get the means and standard deviations for both groups.

In the Independent-Samples T Test box, you get the results for the t-test. The row to look in
is labeled “Equal variances assumed”.

Look in the section of the box labeled t-test for Equality of Means. The value for t is
–2.885. The test has 18 degrees of freedom. The significance level is .010. Because the
alpha level has been set at .05, all you have to do to see is the test is significant is to see is the
significance level reported in the out is .05 or less. .010 is less than .05, so we can say that
the test is significant.

Because the test is significant, our decision is to reject the null hypothesis and accept the
alternative hypothesis.

The sentence that we are allowed to write as our conclusion is the following: Participants in
the caffeine condition remember significantly fewer words than participants in the placebo
condition, t (18) = -2.89, p < .05.

Notice that at the end of the sentence the researcher has given her statistical justification for
why she made that statement. She has reported the value for t, the number degrees of
freedom for the test, and the significance level of the test. Also notice that the symbols “t”
and “p” are in italics. This is the format we want you to use in reporting the result for a t-test.
Use the same format whether or not the test is significant.
Answers to the sample problem
a. Null hypothesis: Participants in the caffeine condition do not remember significantly
fewer words than participants in the placebo condition.
Alternative hypothesis: Participants in the caffeine condition remember significantly
fewer words than participants in the placebo condition.
b. Decision rule: If t ≤ -1.73, reject HO.
c. t = -2.885
d. Decision: reject HO
e. Participants in the caffeine condition remember significantly fewer words than
participants in the placebo condition, t (18) = -2.89, p < .05.
Problems
1. An investigator predicts that students taking an achievement test in the morning will
perform differently than students taking the same test in the afternoon. Eight students
are assigned to take the test in the morning and 8 students are assigned to take the test
in the afternoon.
Morning Group: 25, 31, 44, 36, 29, 42, 39, 41
Afternoon Group: 22, 25, 24, 27, 33, 28, 31, 34
a. State the null and alternative hypotheses.
b. Write the decision rule for rejecting this null hypothesis.
c. Using an alpha level of .05, test the null hypothesis, and state your decision
about whether or not the researcher should reject the null hypothesis.
d. In APA format, write a conclusion sentence that states what the researcher has
learned from doing the test.
e. What is independent variable in this study. What is the dependent variable in
this study.
f. Which is the experimental condition. Which is the control condition.
2. An investigator theorizes that people who participate in a regular program of exercise
will have lower levels of anxiety than people who do not participate in a regular
program of exercise. To test this idea the investigator randomly assigns 12 subjects
to an exercise program for 10 weeks and 12 subjects to a non-exercise comparison
group. Lower anxiety scores reflect lower levels of anxiety. Please test the
investigator's theory using an alpha level of .05.
Exercise Group: 18, 23, 21, 19, 17, 16, 20, 25, 18, 19, 23, 21
No-Exercise Group: 26, 28, 25, 24, 31, 29, 27, 28, 27, 25, 23, 28
a. State the null and alternative hypotheses.
b. Write the decision rule for rejecting this null hypothesis.
c. Using an alpha level of .05, test the null hypothesis, and state your decision
about whether or not the researcher should reject the null hypothesis.
d. In APA format, write a conclusion sentence that states what the researcher has
learned from doing the test.
e. What is independent variable in this study. What is the dependent variable in
this study.
f. Which is the experimental condition. Which is the control condition.
3. An investigator thinks that people under the age of forty have better vocabularies than
people over sixty years of age. The investigator administers a vocabulary test to a
group of 12 younger subjects and to a group of 31 older subjects. Higher scores
reflect better performance.
Under 40 Group: 31, 36, 33, 41, 37, 40, 36, 29, 31, 34, 41, 38
Over 40 Group: 43, 41, 46, 44, 48, 39, 37, 46, 44, 50, 48, 45
a. State the null and alternative hypotheses.
b. Write the decision rule for rejecting this null hypothesis.
c. Using an alpha level of .05, test the null hypothesis, and state your decision
about whether or not the researcher should reject the null hypothesis.
d. In APA format, write a conclusion sentence that states what the researcher has
learned from doing the test.
e. What is independent variable in this study. What is the dependent variable in
this study.
f. Which is the experimental condition. Which is the control condition.
Print out and hand in the output window for these tests.
Answers – Independent t-test problems
1.
HO: Students taking an achievement test in the morning do not perform differently than
students taking the same test in the afternoon.
H1: Students taking an achievement test in the morning perform differently than students
taking the same test in the afternoon.
Decision Rule: If t ≤ -2.14 or if t ≥ +2.145 , reject HO.
Decision: Reject HO
Conclusion sentence: Students taking an achievement test in the morning perform
differently than students taking the same test in the afternoon, t (14) = 2.75, p < .05.
2.
HO: People who participate in a regular program of exercise do not have lower levels of
anxiety than people who do not participate in a regular program of exercise.
H1: People who participate in a regular program of exercise have lower levels of anxiety
than people who do not participate in a regular program of exercise.
Decision Rule: If t ≤ -1.72 , reject HO.
Decision: Reject HO
Conclusion sentence: People who participate in a regular program of exercise have lower
levels of anxiety than people who do not participate in a regular program of exercise, t
(22) = -5.39, p < .05.
3.
HO: People under the age of forty do not have better vocabularies than people over sixty
years of age.
H1: People under the age of forty have better vocabularies than people over sixty years of
age.
Decision Rule: If t ≤ -1.72, reject HO.
Decision: Fail to reject HO
Conclusion sentence: People under the age of forty do not have better vocabularies than
people over sixty years of age, p > .05. [Note: This is because the results are in the
opposite direction from the prediction of the researcher!]