Table 1.

Descriptive and inferential statistics for the candidate covariates. Descriptive statistics for continuous variables report mean ± standard error for each group. Descriptive statistics for ordinal and categorical variables are reported as tabulated frequencies. Inferential statistics for the 2-group binary classification uses the student t-test for continuous, the χ²-test for categorical and the Cochrane-Armitage χ²-test (denoted χ²_CM) for ordinal variables. Inferential statistics for the 4-group ordinal classification uses Spearman's ρ correlation for continuous and ordinal variables and χ²_CM test for categorical variables.

Variable	Descriptive statistics					Inferential statistics		Proportion imputed
	No PEs	With PEs				2-Group binary classification	4-Group ordinal classification
		Suspect	Definite	Clinical	Total
1. Premorbid IQ	111.44 ± 0.12	104.65 ± 0.31	106.79 ± 0.32	103.13 ± 0.42	105.07 ± 0.12	t = 3.456 p = 0.001⁎⁎	ρ = − 0.188 p = 0.003⁎⁎	0.137
2. CIS-R score	6.30 ± 0.05	9.16 ± 0.17	12.26 ± 0.18	16.78 ± 0.34	12.33 ± 0.08	t = − 5.903 p = 1.2 × 10− 8⁎⁎	ρ = 0.408 p = 2.3 × 10− 11⁎⁎	0.065
3. Parental social class								0.173
I	10	4	5	0	9	χ2CM = 9.229, p = 0.002⁎⁎	ρ = 0.204, p = 0.001⁎⁎
II	54	10	16	5	31
III (non-manual)	52	24	23	20	67
III (manual)	6	0	0	2	2
IV	3	5	2	3	10
V	0	1	1	2	4
4. Highest maternal education								0.065
None	11	8	5	5	18	χ2CM = 9.022, p = 0.003⁎⁎	ρ = − 0.197, p = 0.002⁎⁎
Vocational	6	5	3	1	9
Ordinary Level	39	16	21	19	56
Advanced Level	34	7	7	4	18
Degree	35	8	11	3	22
5. Birth weight (g)	3466.2 ± 4.0	3323.4 ± 12.6	3401.7 ± 9.7	3165.3 ± 18.5	3312.2 ± 4.3	t = 2.335, p = 0.020⁎	ρ = − 0.155, p = 0.015⁎	0.060
6. Resuscitated at birth								0.387
No	106	33	38	27	98	χ2 = 1.538, p = 0.463	χ2CM = 0.141, p = 0.707
Yes	19	11	9	5	25
7. Stressful life events								0.173
No	111	36	40	23	99	χ2 = 3.301, p = 0.069	χ2CM = 4.451, p = 0.035⁎
Yes	14	8	7	9	24
8. Handedness								0.411
Right	92	36	34	22	92	χ2CM = 0.001, p = 0.971	ρ = 0.022, p = 0.737
No dominance	25	6	9	7	22
Left	8	2	4	3	9
9. aTobacco consumption (cigarettes per day)	0.59 ± 0.02	1.27 ± 0.08	0.44 ± 0.04	2.23 ± 0.14	1.20 ± 0.03	t = − 1.717, p = 0.087	ρ = 0.021, p = 0.737	0.472
10. bCannabis consumption								0.411
Never	87	28	32	19	79	χ2CM = 1.588, p = 0.208	ρ = 0.077, p = 0.226
Once or twice	11	6	4	1	11
Less than monthly	25	6	8	10	24
Monthly or more	2	4	3	2	9
11. cAlcohol consumption								0.020
Never	16	6	2	0	8	χ2CM = 1.045, p = 0.307	ρ = 0.053, p = 0.408
Once or twice	23	5	9	6	20
Less than monthly	12	5	9	6	20
Monthly	52	21	19	14	54
Weekly or more	22	7	8	6	21
12. dMonth of birth	2.30 ± 7.52	1.72 ± 7.33	1.31 ± 7.50	2.18 ± 7.51	1.71 ± 7.44	F = 1.565, p = 0.212	ρ = 0.088, p = 0.384	0
Standard covariates (included in all analyses)
Age (years)	20.11 ± 0.004	20.14 ± 0.013	19.88 ± 0.011	20.04 ± 0.015	20.01 ± 0.004	t = 1.488, ρ = 0.138	ρ = − 0.147, p = 0.020⁎	0
Gender								0
Male	49	14	16	7	37	χ2 = 2.2757, p = 0.131	χ2CM = 2.952, ρ = 0.085
Female	76	30	31	25	86

^aThe number of cigarettes participant smokes every day on average in the last 30 days.

^bFrom a multiple-choice question asking how many times the participant used cannabis in the past 12 months (all substance use data gathered at age 17/18).

^c6 or more units of alcohol in the past year.

^dAs a scale from 0 (1st January) to 12 (31st December). A Watson-Williams 2-sample F-test and a circular-linear. ρ-correlation using the circular statistics toolbox (Berens and Baclawski, 2009).

^⁎p < 0.05.

^⁎⁎p < 0.01.