Table 3. IQ and academic performance at 18 years of age in EP adolescents compared with controls, and effects of total brain tissue volume on the relationships between groups.

Outcome	Mean score (SD)		Mean difference (95% CI); P-value; r2			% reduction in mean difference#	Relative increase in r2*
	EP (n=147)	Controls (n=131)	Unadjusted	Adjusted†	Adjusted‡
IQ	95.7 (15.9)	107.6 (12.8)	-11.9 (-15.4, -8.5); <0.001; 14.5%	-9.1 (-12.4, -5.8); <0.001; 28.2%	-6.2 (-9.6, -2.7); <0.001; 33.1%	31.9%	17.4%
Reading$	95.5 (13.5)	101.1 (13.6)	-5.6 (-8.8, -2.4); 0.001; 4.1%	-3.7 (-6.8, -0.6); 0.019; 14.3%	-2.2 (-5.3, 1.0); 0.17; 16.1%	40.5%	12.6%
Spelling	97.1 (15.2)	105.1 (14.0)	-8.0 (-11.5, -4.6); <0.001; 7.1%	-6.0 (-9.5, -2.6); 0.001; 16.0%	-4.0 (-7.2, -0.9); 0.012; 18.4%	33.3%	15.0%
Math computation	85.2 (14.0)	95.6 (14.3)	-10.3 (-13.7, -6.9); <0.001; 11.8%	-8.8 (-12.1, -5.6); <0.001; 24.6%	-7.0 (-10.3, -3.8); <0.001; 26.5%	20.4%	7.7%

^$Data available for 146 EP; † adjusted for sex, social class and maternal education; ‡ adjusted for total brain tissue volume, sex, social class and maternal education; # percentage reduction in mean difference between groups when total brain tissue volume was added to the model, calculated as 1 – (the mean difference adjusted for sex, social class and maternal education divided by the mean difference adjusted for total brain tissue volume, sex, social class and maternal education); * percentage increase in r² when total brain tissue volume was added to the model, calculated as (r² in the model adjusted for total brain tissue volume, sex, social class and maternal education minus r² in the model adjusted for sex, social class and maternal education) divided by r² in the model adjusted for sex, social class and maternal education. All P-values are for differences between EP and Control groups.

EP = extremely preterm; r² = % variance explained by the variables in the model; CI = confidence interval; SD = standard deviation.