Table 1

Basic descriptive statistics of the UK Twins sample showing available sample size for each phenotype

Phenotype	N	Mean	Median	Minimum	Maximum	SD
Age, years	2256	53.39	53.02	31.88	73.80	8.19
Height, cm	2256	162.22	162.00	142.00	182.00	5.77
Weight, kg	2256	64.93	63.00	43.00	128.00	11.39
BMI, kg/m²	2256	24.74	23.93	16.23	51.39	4.24
Smoking, pack years	1378	4.11	0	0	78	10.21
Manual work, years	2308	3.06	0	0	46	7.55
Disc height^*	904	3.76	4	0	15	2.68
Disc signal^*	906	5.92	6	0	15	3.26
Disc bulge^*	908	2.19	2	0	10	1.79
Anterior osteophytes^*	903	1.49	1	0	13	1.99
LDD summary score^†	898	13.38	12	0	53	7.66
Hip BMD, g/cm²	5162	0.91	0.91	0.60	1.37	0.13
Lumbar spine BMD, g/cm²	912	0.99	0.99	0.56	1.54	0.16

N is the sample size available for each phenotype.
↵* Summary trait score (0–3) over five intervertebral discs.
↵† Summary score of the four trait scores.
BMD, body mineral density; BMI, body mass index; LDD, lumbar degenerative disc disease.