The Effects of Stimulus Presentation on Anxiety
Purpose
The purpose of this experiment was to determine if there was a difference in anxiety between
exposures to a real spider when compared to a picture of a spider.
Methods
Subjects
Twenty-four college aged subjects with a history of arachnophobia were recruited.
Data Collection
The subjects were randomly assigned to one of the following groups: shown a picture of a
spider, shown a real spider. After viewing either the picture or the real spider the subject’s anxiety was
measured.
Measurement of Reliability
Prior to initiating the study, five subjects (not in the experiment), participated in a test-retest
assessment of measurement reliability. Anxiety was measured on two separate days. Reproducibility of
pull-up measurements was analyzed using SPSS (18.0 for Windows) to compute the intraclass
correlation coefficient (ICC) using a two factor mixed effects model and type consistency (McGraw and
Wong, 1996; Shrout and Fleiss, 1979). A high degree of reliability was found with the single measure
ICC = .921 and the averaged measures ICC = .996.
A Priori Statistical Power Analysis
G*Power version 3.1.2 was used to determine sample size using a mean ± sd of 40 ± 6.9 for the
picture group and a mean ± sd of 50 ± 6.9, a 2-tailed t- test and an alpha level of 0.05. To obtain an
estimated power of 80%, 12 subjects per group would be required.
Statistics
All statistical analyses were performed using a commercial software package (SPSS version 18.0
for Windows, SPSS Inc, Chicago, IL). The anxiety for each group (picture, real spider) was analyzed to
determine if the distributions were normal using Kolmogorov-Smirnov goodness-of-fit test and the
Shapiro-Wilk Normality test. An independent t-test was used to compare the effects of the stimulus
(picture, real spider) on anxiety. Alpha was set at p = 0.05. Data are expressed as mean ± SD.
Results
The effects of the stimulus presentation on anxiety are shown in Table 1. Tests of normality
indicated that differences in anxiety between the two stimulus presentations was normally distributed.
Levine’s test for equality of variance indicated that the groups had equal variance. As shown in Table 1,
there was no difference in anxiety between the two methods of exposure to a spider [t(22) = −1.681, p =
.107]. The stimulus had a medium effect size of 0.34 [Reference Cohen].
Table 1. Effects of stimulus presentation on anxiety (Mean ± SD)
Anxiety (units?)
*
Picture of a Spider
40.00 ± 9.29
Real Spider
47.00 ± 11.03*
p < 0.05; independent t-test.
Conclusion
Arachnophobia individuals experience the same level of anxiety when shown a real spider as
they do when shown a picture of a spider.
References
Cohen J. Statistical Power Analysis for the Behavioral Sciences, Lawrence Erlbaum Associates: Hillsdale,
NJ, 1988.
McGraw KO and Wong SP. Forming inferences about some intraclass correlation coefficients.
Psychological Methods 1: 30-46, 1996.
Shrout PE and Fleiss JL. Intraclass correlations: Uses in assessing reliability. Psychol Bull 86: 420-428,
1979.