Table 2.

Overall Magnitude and Persistence of Source Credibility Effects

	Fixed effects			Random effects
Follow-up	d+ or Δ+	95% CI	Q	d+ or Δ+	95% CI	τ
Immediate impact (d+)	0.35	0.29 to 0.41	107.60***	0.42	0.34 to 0.51	0.05***
Delayed impact (d+)	0.21	0.15 to 0.27	82.08***	0.23	0.18 to 0.28	0.03***
Impact change (Δ+)	−0.15	−0.19 to −0.11	161.87***	0.20	−0.28 to −0.12	0.06***

Note. N = 4502. κ = 53 studies. d₊ and Δ₊ = weighted average effect size; CI = confidence interval around d₊.or Δ₊; τ = estimated between-studies variance of effect parameters. In the first two rows of the table, positive effect sizes indicate that messages attributed to highly credible sources induced greater persuasion than did messages attributed to less credible sources. Effect sizes in the last row represent the extent to which the immediate effect of source credibility changed over time; these effect sizes were computed by subtracting the magnitude of source credibility effects at the immediate post-test from the delayed post-test (Δ). These effect sizes are all negative because the impact of source credibility decayed over time, in general. Model-fit statistic Q is an index of homogeneity of effect sizes included in d₊ or Δ₊. Significant Q values indicate violations of the homogeneity assumption.

^***p < .01.