Table 2.

Intraclass correlation coefficients (ICC*), classic heritability estimates (h^{2 †}) and common environmental variance (CEV^‡) for sigma power and spindle density from eight spindle detector algorithms in monozygotic (MZ) and dizygotic (DZ) twin pairs, adjusted^§

Characteristic	Variable	ICC_MZ	ICC_DZ	h2	CEV
Total power	S0	0.59	0.51	0.16	0.43
Average sigma	S1	0.76	0.58	0.37	0.39
Relative sigma	S2	0.81	0.43	0.77	0.04
	Detector
Density	a2	0.78	0.37	0.83	-0.05
	a3	0.81	0.44	0.72	0.08
	a4	0.57	0.53	0.08	0.49
	a5	0.66	0.30	0.71	-0.05
	a7	0.77	0.38	0.78	-0.01
	a8	0.75	0.16	1.18	-0.43
	a9	0.77	0.40	0.75	0.02
	a10	0.76	0.34	0.83	-0.08

Heritability is calculated using Falconer’s method; values closer to one indicate a pattern expected where variance in a trait is genetically controlled. Negative CEV values suggest that common environmental effects are likely not very important and that genetic dominance effects may be present.

*ICC = σ ²B/[σ ²W + σ ²B].

^†  h  ², heritability = 2(ICCMZ – ICCDZ).

‡ CEV = (2 × ICCDZ) – ICCMZ.

^§Adjusted for age, sex, and age by sex interaction.