Meta-analytic methods for pooling rates when follow-up duration varies: a case study

Table 1 Characteristics of Studies Reporting on School Absences.*

Study	Intervention Group			Control Group			Duration (Months)	Standardized Effect Size**
	N	Mean ± SD	Rate	N	Mean ± SD	Rate
Charlton	42	2.10 ± 11.40	0.18	37	4.70 ± 15.50	0.39	12	-0.19
Christiansen	27	2.39 ± 2.90	0.20	15	2.98 ± 3.29	0.25	12	-0.19
Colland	45	0.98 ± 1.56	0.16	34	0.53 ± 1.08	0.09	6	0.32
Dahl	9	0.80 ± 0.32	0.8	10	0.90 ± 0.32	0.9	1	-0.30
Deaves	32	3.69 ± 4.80	0.31	31	5.19 ± 4.80	0.43	12	-0.31
Evans	117	19.40 ± 13.90	1.62	87	19.70 ± 12.60	1.64	12	-0.02
Fireman	13	0.50 ± 5.06	0.04	13	4.60 ± 5.06	0.38	12	-0.78
Hill	211	5.43 ± 4.07	1.36	193	6.23 ± 4.72	1.56	4	-0.18
Hughes	44	10.70 ± 6.90	0.89	45	16.00 ± 15.40	1.33	12	-0.44
Mitchell	133	7.92 ± 16.48	1.32	126	8.48 ± 26.69	1.41	6	-0.03
Perrin	29	0.24 ± 0.90	0.24	27	0.22 ± 1.00	0.22	1	0.02
Persaud	18	6.40 ± 4.60	1.28	18	7.60 ± 5.30	1.52	5	-0.24
Rubin	29	11.90 ± 7.80	0.99	25	15.40 ± 15.00	1.28	12	-0.30
Talabere	25	1.36 ± 2.52	0.45	25	2.60 ± 3.75	0.87	3	-0.38
Toelle	63	2.62 ± 3.28	0.44	51	2.67 ± 3.21	0.45	6	-0.02
Wilson	30	0.80 ± 2.29	0.80	29	1.40 ± 3.23	1.40	1	-0.21

* N refers to the sample size, Mean ± SD refers to the mean number of events ± standard deviation, and rate refers to the total events per person-month. ** Standardized effect size was calculated for each study by subtracting control group mean from intervention group mean and dividing by the pooled SD.

ISSN: 1471-2288