Question

  Do positive or negative messages have a greater effect on behavior? Forty-two subjects were randomly assigned to one of two treatment groups, 21 per group. In one group, subjects read an introduction about a nonprofit organization followed by a negative message encouraging subjects to sign a petition for cleaner lakes. In the other group, subjects read the same introduction followed by a positive message encouraging subjects to sign the petition. All subjects were then tested to determine how strongly they intended to sign the petition, with higher scores indicating greater intent. The 21 subjects receiving the negative message had a mean score of 9.64 with standard deviation 3.43; the 21 subjects receiving the positive message had a mean score of 15.84 with standard deviation 8.65.

At ? = 0.05, is there enough evidence to say that a negative message results in a lower mean score?

Answer 1

Do positive or negative messages have a greater effect on behavior? Forty-two subjects were randomly assigned to one of two treatment groups, 21 per group. In one group, subjects read an introduction about a nonprofit organization followed by a negative message encouraging subjects to sign a petition for cleaner lakes. In the other group, subjects read the same introduction followed by a positive message encouraging subjects to sign the petition. All subjects were then tested to determine how strongly they intended to sign the petition, with higher scores indicating greater intent. The 21 subjects receiving the negative message had a mean score of 9.64 with standard deviation 3.43; the 21 subjects receiving the positive message had a mean score of 15.84 with standard deviation 8.65.

At ? = 0.05, is there enough evidence to say that a negative message results in a lower mean score?

Lower tail test

t = -3.0533 

DF = n₁+n₂-2 =40

Table value of t with 40 DF at 0.05 level = -1.6839

Rejection Region: Reject Ho if t < -1.6839

Calculated t = -3.0533, falls in the rejection region

The null hypothesis is rejected.

There is enough evidence to say that a negative message results in a lower mean score.

Pooled-Variance t Test for the Difference Between Two Means
(assumes equal population variances)
Data
Hypothesized Difference	0
Level of Significance	0.05
Population 1 Sample
Sample Size	21
Sample Mean	9.64
Sample Standard Deviation	3.43
Population 2 Sample
Sample Size	21
Sample Mean	15.84
Sample Standard Deviation	8.65
Intermediate Calculations
Population 1 Sample Degrees of Freedom	20
Population 2 Sample Degrees of Freedom	20
Total Degrees of Freedom	40
Pooled Variance	43.2937
Standard Error	2.0306
Difference in Sample Means	-6.2000
t Test Statistic	-3.0533
Lower-Tail Test
Lower Critical Value	-1.6839
p-Value	0.0020
Reject the null hypothesis