Characteristics of meta-analyses and their component studies in the Cochrane Database of Systematic Reviews: a cross-sectional, descriptive analysis

Table 5 Number of studies per meta-analysis, overall and broken down by outcome type, intervention comparison type and medical area

		Meta-analyses	50%¹	75%	90%	99%	Max
All	Total	22453	3	6	10	28	294
Data type	Dichotomous	14886	3	6	10	28	294
	Continuous	6672	3	5	8	24	98
	Mixed	895	4	7	12	46	133
Medical specialty	Cancer	905	5	8	14	43	69
	Cardiovascular	1693	4	7	13	38	58
	Central nervous system/musculoskeletal	1965	3	5	8	21	49
	Digestive/endocr., nutritional and metabolic	2704	4	6	11	31	138
	Gynaecology, pregnancy and birth	4656	3	5	8	20	76
	Infectious diseases	965	3	5	9	27	63
	Mental health and behavioural conditions	2918	3	5	10	30	174
	Pathological conditions, symptoms and signs	701	3	6	10	31	294
	Respiratory diseases	3021	3	5	8	20	39
	Urogenital	1289	3	5	9	21	31
	Other	1636	3	6	10	34	133
Interventions	Pharm. vs. Control/Placebo	8348	3	6	10	30	294
	Pharm. vs. Pharm.	5571	3	6	10	28	174
	Pharm. vs. Non-Pharm.	424	3	5	11	25	36
	Non-Pharm. vs. Control/Placebo	4449	3	5	9	25	61
	Non-Pharm. vs. Non-Pharm.	3661	3	5	9	28	58
Outcomes	Signs/symptoms reflecting continuation/end of condition	3639	3	5	9	29	110
	Adverse events	2368	3	6	10	26	270
	Infection/onset of new acute/chronic disease	2215	3	6	10	24	294
	Biological markers	2071	3	6	10	28	138
	General physical health	1921	3	5	9	27	58
	Obstetric outcomes	1477	3	6	11	24	76
	All-cause mortality	1278	4	8	13	36	75
	Resource use/hospital stay/process	1187	3	5	9	27	99
	Cause-specific mortality/major morbidity event/composite (mortality or morbidity)	920	4	6	11	34	133
	Other outcomes (semi-objective)²	2044	3	6	11	30	174
	Other outcomes (subjective)³	3239	3	5	9	23	69

¹Minimum values and 25^th percentiles are not listed, since these were identical across most meta-analysis types. The minimum value was 2 in all rows (noting that meta-analyses needed to contain at least 2 studies to be eligible for inclusion). The 25^th percentile was 3 for Cancer and All-cause mortality, and 2 in every other row.
²Other outcomes (semi-objective): External structure, Internal structure, Surgical/device related success/failure, Withdrawals/drop-outs
³Other outcomes (subjective): Pain, Mental health outcomes, Quality of life/functioning, Consumption, Satisfaction with care, Composite (at least 1 non-mortality/morbidity)

ISSN: 1471-2288