Table 1.

Means, standard deviations and ANOVA results by sex.

Measures	Means and standard deviations for raw data including outliers		Means and standard deviations for standardised data						ANOVA-effects of sex
	All		All		Females		Males		Sex
	M (n)	SD	M (n)	SD	M (n)	SD	M (n)	SD	p	η2
1 GCSE	8.93 (n = 5274)	9.00	0.04 (n = 5218)	0.96	0.01 (n = 2772)	0.97	0.07 (n = 2446)	0.95	0.02	0.00
2 Maths Web test composite	–	–	0.02 (n = 2303)	1.00	− 0.13 (n = 1350)	0.99	0.24 (n = 953)	0.98	0.00	0.03
3 Academic Self-concept	3.56 (n = 2215)	3.60	0.01 (n = 2210)	0.99	− 0.12 (n = 1301)	1.01	0.20 (n = 909)	0.92	0.00	0.03
4 Maths Self-Efficacy	17.67 (n = 2403)	19.00	0.02 (n = 2385)	0.96	− 0.17 (n = 1391)	0.99	0.30 (n = 994)	0.85	0.00	0.06
5 Maths Interest	2.54 (n = 2404)	2.67	0.01 (n = 2404)	1.00	− 0.09 (n = 1405)	1.03	0.14 (n = 999)	0.94	0.00	0.01
6 Classroom Environment	32.34 (n = 2391)	33.00	0.02 (n = 2381)	0.98	− 0.01 (n = 1388)	1.02	0.06 (n = 993)	0.93	0.07	0.00

M = mean; SD = standard deviation; n = sample size; p = p-value of the effects of sex on variables; η² = eta-squared. Means and standard deviations reported on the standardised scores refer to variables correct for age and cleared of outliers scores (± 3 standard deviations). Mean and standard deviation for the Maths web composite is not provided as this is derived averaging the standardised means of the two mathematics web tests. The table presents results of analyses conducted on half of the sample constituted by one randomly selected twin in each pair. Sample size is larger for GCSE scores because data was collected on the 3 cohorts, while for all the other measures, the web data was collected only on 2 cohorts.