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Table 1 Potential confounders, their categories and internal drop-outs

From: The influence of self-reported leisure time physical activity and the body mass index on recovery from persistent back pain among men and women: a population-based cohort study

Potential confounder Categories Internal drop-outs aM/W (%)
Smoking habits Daily smoker, Not daily smoker 0/1
Alcohol consumption (Grams of 100% alcohol/day) No alcohol consumption, 6/5
Low (males >0 to 40 g/day, females >0 to 20 g/day),
Moderate (males >40 to 60 g/day, females >20 to 40 g/day),
High (males >60 g/day, females >40 g/day)
Neck pain the latest 6 months No pain, Two days in total, On average two days a month, 1/1
On average two days a week, Every day
Chronic illness or handicap Suffering from long lasting disease, handicap or disability from accidental event? (Yes/No) 2/2
Psychological wellbeing (GHQ-12) b Reduced psychological wellbeing, 2/1
Not reduced psychological wellbeing
Emotional social support c I have emotional social support, 1/1
I do not have emotional social support
Instrumental social support d I have instrumental social support, 1/1
I do not have instrumental social support
Socioeconomic class (SEI class) e Unskilled and semiskilled workers, 4/5
Skilled workers,
Assistant non-manual employees,
Intermediate non-manual employees,
Employed/self-employed professionals/ higher civil servants and executives,
Self-employed (other than professionals)
Current occupation Employed, Self-employed, Unemployed, 3/4
On disability or retirement pension,
On leave, Student, “Taking care of the household”
Marital status Married/registered partnership, Unmarried, 0/0
Divorced, Widow or widower
Birthplace Born in Sweden, Born abroad 0/1
Time spent doing housework a normal weekday Almost no time at all, About 30 minutes, 2/1
1-2 hours, 3-5 hours, > 5 hours
Main physical work load the latest 12 months f Sedentary, Light (mobile but no heavy lift), 48/51 h
Moderate (mobile and some heavy lift), Heavy
Sick leave during the latest 12 months No sick leave, 1-7 days, > 8 days 38/48 h
Freedom to decide how to perform work g Never, Mostly not, Mostly, Always 42/51 h
Freedom to decide what to perform at work g Never, Mostly not, Mostly, Always 43/51 h
Good collegiality at work g No, Not so good, Quite good, Good 43/51 h
  1. Note: Study population (n=1836) with 632 men (M) and 1204 women (W).
  2. a Proportion of missing answer in the potential confounder covariate (%). M = Men, W = women.
  3. b GHQ-12: the twelve-item general health questionnaire. GHQ-12 is regarded as valid to detect cases with reduced psychological wellbeing. Gender, age and education level have no significant effect on the validity [19]. GHQ-12 is described by McDowell [20].
  4. c Emotional social support: Assess whether participants have persons who can support them in handling personal problems or critical life events.
  5. d Instrumental social support: Assess whether participants have persons who can support them in practical matters when they are sick or have practical problems. The social support questions originates from an instrument (ISSI) described by McDowell and tested, as a short form (SS-13), by Undén and Orth-Gomer in a population of Swedish industrial workers [20, 21].
  6. e SEI class: Based on occupation and education [22].
  7. f Main physical work load the previous twelve months: For working men and women with musculoskeletal complaints, this question was found to have moderate criterion validity, compared to a structural personal interview (weighted kappa coefficient of 0.68 [95% CI: 0.60-0.76]) [17].
  8. g Freedom to decide how to perform work, Freedom to decide what to perform at work and good collegiality at work: Questions included in the Swedish Demand-Control-Support Questionnaire (DCSQ) [23].
  9. h High proportion of internal dropouts partly due to many participants not working.