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

From: Self-reported hard physical work combined with heavy smoking or overweight may result in so-called Modic changes

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.