Table 5

Results from a linear regression model showing number of daily defined doses (DDD) for all-purpose drugs according to educational status and income in a population of Danish twins (n=24 370) stratified on zygosity

	Unpaired analysis*				Intrapair analysis†
Educational status	DZSS n=15 788		MZ n=8582		DZSS		MZ
Income	DDD	95% CI	DDD	95% CI	DDD	95% CI	DDD	95% CI
Secondary	−287	−397 to −177	−289	−440 to −138	−245	−399 to −90	−143	−346 to 59
Tertiary	−404	−531 to −277	−424	−606 to −243	−344	−557 to −131	−152	−440 to 136
2nd quartile	−401	−541 to −260	−275	−463 to −88	−219	−383 to −54	18	−177 to 214
3rd quartile	−541	−680 to −402	−451	−638 to −263	−367	−535 to −198	−83	−287 to 121
4th quartile	−505	−648 to −363	−437	−630 to −243	−327	−512 to −142	16	−209 to 242

Results from unpaired and intrapair analyses are shown.
*Unpaired analysis treating twins as individuals while taking interdependence of observations within each individual and within twin pairs into account. The interpretation of, for example, the education estimate is difference in the mean number of DDDs of an individual with a secondary or tertiary education compared with a random individual with a primary education. The estimates are adjusted for age and sex. Income and education are mutually adjusted.
†Intrapair analysis of twins. The interpretation of for example, the education estimate is the difference in the mean number of DDDs of a twin with a secondary or tertiary education compared with its co-twin with a primary education.
Italic typography indicates statistical significance at 5% level.
DZSS, dizygotic same sex twins; MZ, monozygotic twins.