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Journal of Consulting and Clinical Psychology
1996. Vol.64, No. 3, 517-519
Copyright 1996 by the American Psychological Association, Inc.
0022-006X/96/S3.0 0
Hypnoti c Enhancemen t of Cognitive-Behaviora l
Weight Loss Treatments—Anothe r Meta-Reanalysi s
Irvin g Kirsc h
University of Connecticut
In a 3rd meta-analysis of the effect of adding hypnosis to cognitive-behavioral treatments for weight
reduction, additional data were obtained from authors of 2 studies, and computational inaccuracies
in both previous meta-analyses were corrected. Averaged across posttreatment and follow-up assess-
ment periods, the mean weight loss was 6.00 Ibs. (2.72 kg) without hypnosis and 11.83 Ibs. (5.37 kg)
with hypnosis. The mean effect size of this difference was 0.66
SD.
At the last assessment period, the
mean weight loss was 6.03 Ibs. (2.74 kg) without hypnosis and 14.88 Ibs. (6.75 kg) with hypnosis.
The effect size for this difference was 0.98
SD.
Correlational analyses indicated that the benefits of
hypnosis increased substantially over time (
r =
.74).
Meta-analyses allow comparisons of outcomes amongstudies
using different instruments to measure dependent variables.By
standardizing scores, the effects of psychotherapy (Smith,
Glass, &Miller, 1980) or of the addition of particular therapeu-
tic procedures to therapy (Kirsch, Montgomery, & Sapirstein,
1995) can be assessed across a wide range of presenting prob-
lems. Even in studies assessing treatment effects on the same
presenting problem (e.g., depression), the use of different mea-
suring instrumentsmay require the calculation of standardized
effect sizes. The studies comparing weight reduction treatments
with and without hypnosis present less of a problem. Weight
loss in pounds or kilogramswere reported in each of them. Be-
cause they used a common dependent measure, their resultscan
be examined directly without having to decide what assump-
tions should be made in estimating unreported parameters.
1
The mean weight loss in hypnotic treatments, nonhypnotic
treatments, and the difference in mean weight loss between
these two forms of treatment are presented in Table 1.
2
Across
all assessment periods, these data indicate a mean weight loss of
6.00 Ibs. (2.72 kg) without hypnosis and 11.83 Ibs. (5.37 kg)
with hypnosis. Thus, includinghypnosis in the treatment pro-
tocol resulted in an additional loss of 5.83 Ibs. (2.64 kg), a97%
increase in treatment efficacy.
The effect of treatment for many presentingproblems should
be readily apparent by the end of treatment. The purpose of
follow-up assessments in these cases is to evaluate the durability
of treatment effects. Weight loss treatments are somewhat
different in this respect. Their aim is to produce a change in
eating and exercise habits, the results of which are revealed
gradually in weight change. Because the effects of these treat-
ments on weight are not fully apparent at the conclusion of
treatment, averagingacross assessment periods may underesti-
I thank Philip Bornstein and Marianne Barabasz for supplying addi-
tional data for this reanalysis and Robert Nadon for his very helpful
suggestions and advice.
Correspondence concerning this article should be addressed to Irving
Kirsch, Department of Psychology, U-20, University of Connecticut,
406 Babbidge Road, Storrs, Connecticut 06269-1020.
mate the actual effect of a weight loss treatment. Alternately,it
may overestimate treatment effects if there is a return to previ-
ous eating habits. In either case, weight loss at the final assess-
ment period is the more accurate estimate of treatment effects.
As shown in Table 1, mean weight loss at the final assessment
was 6.03 Ibs. (2.74 kg) without hypnosis and 14.88 Ibs. (2.75
kg) with hypnosis. Thus, including hypnosis in the treatment
protocol resulted in an additional loss of 8.85 Ibs. (4.01 kg), a
147% increase in treatment efficacy.
These means are based on simple calculations from data re-
ported in the treatment studies and are not in dispute. The
question is, howcould such a largedifference in weightloss pro-
duce the smal effect size reported by Allison and Faith (1996)?
To answer this question, I recalculated effect sizes using addi-
tional information obtained from authors of the studies. The
results of these analyses are presented in Table 1.
Method
Known Standard Deviations
Posttreatment standard deviations were reported by Deyoub and Wil-
kie, (1980). Because follow-up standard deviations were not reported,
I used the pooled posttreatment standard deviation in calculating both
1
To facilitate comparisons across studies, all data are reported in
pounds.
2
Like Kirsch et al. (1995), Allison and Faith (1996) found significant
enhancement of treatment effects that was due to the addition of hyp-
nosis. However, noting that there were typographical errors in one of the
studies (Goldstein. 1981) and that effect size estimates in that study
might be confounded because participants were allowed to come for
additional treatment sessions, they characterized the study as "ques-
tionable" and recalculated the mean effect size without it. With
Goldstein's (1981) study excluded, they reported a nonsignificant effect
for the addition of hypnosis to treatment. Toaddress this concern, I have
excluded the Goldstein study from all analyses.
517