The simple Cox Proportional Hazard Model used to estimate the unadjusted association…

The simple Cox Proportional Hazard Model used to estimate the unadjusted association between contraception discontinuation and injection administrator (self vs. provider) is as follows: ln(hazard of discontinuation at time t) = ln(lambda_o_hat (t)) + -0.62×1 where x1=1 for those who self-inject the contraceptive and 0 for those who have a provider inject the contraceptive. The standard error of the slope for x1 = 0.11. The researchers also presented adjusted hazard ratios (adjusted for education level, spousal approval of contraceptives, and other factors) separately for those younger than 25 years old, and those greater than or equal to 25 years old. The resulting hazard ratios, and 95% CIs are as follows: < 25 years old: 0.40 (0.32, 0.51) >= 25 years old 0.75 (0.56, 0.99) The best explanation for these results is: