Table 3. Comparison of models to explain the association of postnatal age with carboxyhemoglobin levels.

Model and variables†	Akaike Information Criterion	Bayesian Information Criterion	Log likelihood	Chi- squared	P (for indicated comparisons)
0. Intercept only (null)	1211	1225	-603	Reference
1. Age	1013	1031	-502	201	< 2.2 x10-16 vs. model 0
2. ln (Age)	891	909	-441	121	< 2.2 x10-16 vs. model 0
3. ln (Age) + (ln (Age))2	852	875	-421	41.1	1.48x10-10 vs. model 2
4. ln (Age) + (ln (Age))2 + (ln (Age))3	814	841	-401	40.2	2.35x10-10 vs. model 3

^†All models include subject identity as a random factor.