Table 3

Variables of equation 2, estimating the probability for a male or female infant of needing treatment for IHPS

	β, male	SE	p value	β, female	SE	p value
β₀, constant	+1.97289	1.35467		+3.36770	2.91945
Y, days after 31 Dec 1986	+0.00064	0.00074	0.3851	−0.00125	0.00147	0.3944
Z, latitude, degrees	−0.12833	0.02315	<0.0001	−0.17793	0.04999	0.0004
Y·Z, (days) · (latitude)	−0.00001	0.00001	0.2878	+0.00002	0.00003	0.4210

The interaction term, Y·Z, was used in order to specifically investigate whether variation in incidence with time (Y) is related to latitude (Z).