Table 2 A description of how the variables of interest were defined and grouped.

1. Outcome variables

1. LBP some time in the past year, "LBPyear" (yes/no)

2. LBP for altogether at least 30 days in the past year, "LBPlong" (yes/no)

3. Disc degeneration, "DD" (yes/no)

4. Disc degeneration with VIP, "VIP" (yes/no)

2a. Single explanatory variables

1. "Smoking", divided into 0, 1–19 cigarettes/day, and ≥20 cigarettes/day.

2. "Body mass index" (BMI) divided into BMI <20 ("underweight"), BMI 20–24 ("normal weight"), BMI 25–29 ("overweight"), and BMI >29 ("heavy overweight").

3. "Work", divided into mainly sitting, sitting/walking, light physical work, and hard physical work

2b. combinations of the explanatory variables

Based on the results obtained with the single explanatory variables, the following variables were created:

1. "Heavy smoking" (yes/no); i.e. smoking divided into ≥20 cigarettes a day vs. 0–19 cigarettes/day.

2. "Hard physical work" (yes/no); i.e. hard physical work vs. the other three types.

3. "Overweight" (yes/no); i.e. overweight and heavy overweight vs. normal weight and underweight.