Figure. Adjusted log-odds of white matter hyperintensity progression and pack-years of smoking, parameterized using penalized splines.

Adjusted log-odds of white matter hyperintensity progression across the range of pack-years of smoking among current (A), former (B), and ever (C) smokers at the time of the initial MRI. Rug plots demonstrate the distribution of pack-years of smoking in the sample. The log-odds at each value of pack-years of smoking are denoted by a solid black line, with gray shading indicating the 95% confidence intervals for the estimated log-odds. The association for a given difference in pack-years (e.g., 20–30) is derived through comparison of the log-odds at each of these 2 points. A straight line indicates that the association (here, the odds ratio) for a given absolute difference in pack-years is the same regardless of the actual values chosen as the starting point. A curved line suggests that that magnitude and potentially also direction of the association for a given difference in pack-years varies depending on the starting pack-years value.