What can be learnt from models of incidence rates?

Breast Cancer Research

Table 1 Parameters for estimated rate of tissue aging from the Pike incidence model and Rosner extended model

		Rosner, Colditz and Willett [19]
Rate of tissue aging	Pike et al. [18]	One-birth model	Two births	Multiple births
Menarche to first birth	1.0	1.0	1.0	1.0
After first birth, before menopause	0.70	0.67 (p < 0.001)	0.84 (p = 0.19)	0.79 (p = 0.002)
After second birth			0.68 (p = 0.20)	0.03^a (p = 0.02) (per year from birth to menopause, for each birth)
After menopause	0.105	0.43 (p = 0.002)	0.43 (p = 0.002)	0.46^b(p = 0.002)
Increase at first birth (years)	2.2	7.45 (p < 0.001)	6.4 (p = 0.02)	7.61 (p < 0.001)
Increase at second birth			-0.02 (p = 0.99)	-1.69
K	Fixed at 4.5	2.9 (2.5–3.3)	2.8 (2.4–3.2)	2.8 (2.4–3.2)

^aFor the multiple birth model this represents the decrease in the rate of tissue aging per year with each subsequent birth after the first, so for births at 27, 30 and 33 with menopause at age 50 the cumulative rate of tissue aging is decreased by 1.11 units.^bAssumes three births. The log likelihood for multiple births model is significantly better than the one-birth model (χ², 2 df 17.82, p < 0.001).

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