. Author manuscript; available in PMC: 2011 Aug 25.

Published in final edited form as: Pediatrics. 2009 Feb;123(2):669–673. doi: 10.1542/peds.2008-1117

TABLE 4.

Testing Independence and Concordance of the Twins

Variable	Expected Concordance	Observed Concordance	P
Independence			<.001
Monozygotic	2.378^a	13
Dizygotic	8.000^a	32
Excess of concordance of the monozygotic twins			.34
Monozygotic	10.3^b	13
Dizygotic	34.6^b	32

The expected concordance was calculated under the assumption that twins are independent, the rate of PDA is [10 + (2 × 13) + 34 + (2 × 32)]/864 = 0.155, and the expected concordance for PDA is 99(0.155)² for monozygotic twins and 333(0.155)² for dizygotic twins.

The expected concordance was calculated under the assumption that the monozygotic and dizygotic twins have the same level of concordance that is estimated by (13 + 32)/432 = 0.104, and the expected concordance for PDA is 99 × 0.104 for monozygotic twins and 333 × 0.104 for dizygotic twins.