Table 4.

Threshold effect analysis of WWI on myopia using a two-piecewise linear regression model

	Male	Female
Fitting by standard linear model
OR (95% CI)	1.026 (0.916, 1.149)	0.907 (0.824, 0.988)
P value	0.656	0.046
Fitting by two-piecewise linear model
Breakpoint (K)	10.774	10.025
WWI < K	0.938 (0.818, 1.077) 0.366	0.679 (0.512, 0.899) 0.007
WWI > K	1.398 (1.038, 1.884) 0.028	0.987 (0.876, 1.112) 0.829
Logarithmic likelihood ratio test P value	0.022	0.027

Sex, age, race, education level, body mass index, serum vitamin D level, poverty income ratio, and TV and computer use time were adjusted

The model is not adjusted for the stratification variable itself

OR: odds ratio; 95% CI: 95% confidence interval; WWI: weight-adjusted-waist index