Table 1.

Studies included in the systematic review of premorbid intelligence and Schizophrenia.

Study	Design and setting	Diagnostic criteria and outcome	Case (n)	Control (n)	% of cases male	Age at follow up (years)	IQ test age (years)	Mean age of onset, years (SD)	IQ tests	Case identification	Adjustment for confounding
Jones et al. (1994)	PCa—British 1946 birth cohort	DSM III R schizophrenia	24	4133	66.6	43	8, 11, 15	23.4 (2.7)	1st principal componentb	Questionnaire, interview, hospital discharge register, present state examination	Age, social class
Crow et al. (1995)	PC—British 1958 birth cohort	PSE CATEGOc schizophrenia	29	1446	51.7	28	7, 11, 16	–	General ability test	Hospital discharge register	Social class
Cannon et al. (2000)	PC—NCPP: Philadelphia cohort, USAd	DSM IV Schizophrenia or schizoaffective	57	5829	72	19–36	4, 7	–	Wechsler intelligence scale for children (WISC)	Hospital discharge register	Socio-demographic, obstetric, behavioral factors
Walker et al. (2002)	RCe—Scottish mental ability survey 1932	ICD10: F20–29 Schizophrenia and related conditions	32	3764	65.5	66	11	36.5 (16.6)f	Moray house test	Hospital discharge register	Age
Cannon et al. (2002)	PC—Dunedin birth cohort, New Zealand	DSM IV Schizophreniform	33	598	–	32	3, 5, 7, 9, 11	–	Revised Wechsler intelligence scale for children (WISC-R)	Diagnostic interview schedule (DIS)	Socioeconomic, obstetric and maternal factors
Gunnell et al. (2002)	RC—Swedish conscript cohort	ICD 10 schizophrenia	60	109 491	100	18–25	18	20.8 (1.5)f	Synonym and visuospatial test	National Inpatient Discharge Register	Age, obstetric and maternal factors, parental education
Caspi et al. (2003)	NCCg—Israeli conscript cohort	DSM IV schizophrenia	44	44	75	19–31	16–17	22.5 (3.5)f	Modified Raven's progressive matrices, Otis type verbal IQ test	Structured Clinical Interview for DSM (SCID)	Sex, country of origin, age, education
Niendam et al. (2003)h	NCC—NCPP: Philadelphia cohort, USA	DSM IV schizophrenia or schizoaffective	32	201	34	19–36	7	–	Vocabulary and block design	Hospital discharge register	Age, socio-economic status
Zammit et al. (2004)	RC—Swedish conscript cohort	ICD 8 and ICD 9 Schizophrenia	362	49 161	100	45–47	18	28.2 (5.9)f	Visuospatial test	National Inpatient discharge register	–
Reichenberg et al. (2005)	RC—Israeli conscript cohort	ICD 10 Schizophrenia	1856	549 466	76	24–34	16–17	23.8 (3.7)f	Verbal and non-verbal analogies	National Inpatient discharge register	Sex, socioeconomic status
Tiihonen et al. (2005)i	RC—Finnish conscript cohort	ICD 8 and ICD 9 schizophrenia	607	189 027	100	27	19.9	–	Verbal, arithmetic and visuospatial reasoning tests	National inpatient discharge register	Age, over-all test performance
Seidman et al. (2006)	NCC—NCPP: New England cohort, USA	DSM IV schizophrenia	31	61	79.4	28	7	–	Wechsler intelligence scale for children (WISC)	DIS, SCID, hospital discharge register	Sex, ethnicity, socioeconomic status
Osler et al. (2007)	PC—Danish longitudinal study	ICD 10 schizophrenia	87	6790	100	19–49	12	35.2 (9.6)f	Härnquist test	National inpatient discharge register	Birth weight, social class, education, marital status, employment
Urfer-Parnas et al. (2010)	NCC—Danish conscript cohort	ICD 8 to ICD 10 schizophrenia	1779	20 531	100	43–54	19.5	30.4 (7.8)f	Børge Priens Prøve (letter matrices, verbal analogies, number series, geometric figures)	National inpatient discharge register	–

^aPC—Prospective cohort study.

^b1st principal component derived from all tests used at each age which included tests for verbal, nonverbal and mathematical skills.

^cPresent state examination and CATEGO.

^dNational Collaborative Perinatal Project (NCPP)—USA.

^eRC—Retrospective cohort study.

^fAge at the time of first hospital admission for schizophrenia.

^gNCC—Nested case–control study.

^hStudy sample is a sub-set of T Cannon et al. (2000) used in the meta-analysis of premorbid verbal and performance IQ.

ⁱStudy used only in the dose–response analysis as data was not available in appropriate format to use in other analyses.