2. Effect sizes for random‐effects and fixed‐effect analyses, and heterogeneity for random‐effects analyses.

Outcome	N°of studies	N°of participants	Random‐effects model			Heterogeneity			Fixed‐effect model
			SMD (95% CI)	Z	P	Chi2	P	I2 (%)	SMD (95% CI)	Z	P
Mixed/regular word reading accuracy	11	701	0.51 (0.13 to 0.90)	2.59	0.01	52.11	< 0.001	81	0.48 (0.32 to 0.64)	5.78	< 0.001
Non‐word reading accuracy	10	682	0.67 (0.26 to 1.07)	3.24	0.001	50.72	< 0.001	82	0.68 (0.51 to 0.84)	8.03	< 0.001
Irregular word reading accuracy	4	294	0.84 (0.30 to 1.39)	3.04	0.002	14.41	0.002	79	0.82 (0.58 to 1.07)	6.66	0.002
Mixed/regular word reading fluency	4	224	0.45 (–0.19 to 0.72)	3.33	< 0.001	2.20	0.53	0	0.45 (0.19 to 0.72)	3.33	< 0.001
Non‐word reading fluency	3	188	0.39 (0.10 to 0.68)	2.63	0.009	0.02	0.99	0	0.39 (0.10 to 0.68)	2.63	0.009
Reading comprehension	5	343	0.28 (–0.07 to 0.62)	1.54	0.12	8.45	0.08	53	0.23 (0.01 to 0.45)	2.07	0.040
Spelling	3	158	0.47 (–0.07 to 1.01)	1.72	0.09	3.89	0.14	49	0.28 (–0.09 to 0.65)	1.49	0.14
Letter‐sound knowledge	3	192	0.35 (0.04 to 0.65)	2.22	0.03	0.11	0.95	0	0.35 (0.04 to 0.65)	2.22	0.03
Phonological output	4	280	0.38 (–0.04 to 0.80)	1.77	0.08	7.97	0.05	62	0.44 (0.19 to 0.70)	3.45	< 0.001

CI: confidence interval; SMD: standardised mean difference.