The controls were approached in a similar way as were cases and proxies, with invitation letter, questionnaire and telephone reminder if needed.

Among the 480 non-fatal cases who participated in the SHEEP during the study period, 327 were eligible for inclusion. Out of these, 31 either changed their marital status or died before the proxy was recruited in the present sub-study. Among the 296 remaining eligible cases, 53 proxies did not participate. Thus, the analyses are based on 243 case-proxy pairs.

Eligible cases for this study were non-fatal MI cases (survival after MI for at least 28 days) participating in the SHEEP and who received the questionnaire between April 5 th 1993 and December 31 st 1993. In addition, these cases were co-habiting with a spouse or a common-law spouse at the time of the MI event. After each eligible case was identified, the SHEEP secretariat mailed to each eligible case a letter of invitation. The letter included information about the study and asked for permission to contact his/her spouse/common law spouse (proxy). If the case agreed to participate, the proxy was sent a similar letter of invitation. Eligible proxies were also contacted by telephone in order to provide additional information about the study and to seek informed consent to participate. The proxies who agreed to participate were asked to complete a questionnaire similar to the one the MI patients completed. The proxies were asked to complete the questionnaire without help from their spouses, the MI patients. Participants who left questions unanswered were contacted by telephone and were asked the questions again. When the data collection had ended, the proxies were sent a letter where they were asked about whether some parts of the questionnaire were difficult to fill out and whether they received any help from their spouses, the MI patients, in responding to any of the questions. None of the proxies reported they received help from their spouses. However, 5 reported they had some difficulty giving information about work related exposures.

This validation study is based on a subset of individuals included in the SHEEP, a case-control study of risk factors for MI performed in Stockholm County among men and women 45–70 years old who were Swedish citizens and free from previous MI. Details about the SHEEP design have been described in an earlier study [ 9 ]. Briefly, first time MI events were identified in the greater Stockholm area between 1992 and 1994. Over the study period, control individuals (at least one control per case) were continuously (within 2 days from case incidence) and randomly selected from the study population using the Stockholm county population register after matching for sex, age (five-year intervals), and residential area. All cases and controls completed a questionnaire, which included questions about life style, body habitus, environmental exposures, and psychosocial environment. The questions to cases and controls were almost identically formulated. The median time to response among cases was 26 days (interquartile range, IQR: 17–38 days), and among controls it was 39 days (IQR: 26–93 days).

The study was approved by the Ethical Committee at Karolinska Institutet (91:259). All study participants gave their informed oral consent to be enrolled in the study; at the time the study was initiated (1992) forms for written consent were generally not used. An invitation letter was sent by mail to eligible subjects informing about the study. The information included description of rationale for the study, study aims, study design and that participation involved filling out a questionnaire. The letter also stated that participation was voluntary and that confidentiality was guaranteed. The receiver of the invitation was asked to return the completed questionnaire by mail. Employees at the SHEEP central received the questionnaires and contacted each respondent by telephone, asking about clarification of unclear answers and documenting informed consent to participate. A second letter of invitation, a reminder, was sent to those who did not reply to the first letter, and the same procedure for documentation of oral consent was used.

Variable definitions

Traditional cardiovascular exposures. Hypertension was considered present if the participants answered “yes” to the question “do you have hypertension?”/”does your close relative have hypertension?”, or reported use of any medication against hypertension. Identification of diabetes and hyperlipidaemia, respectively, was correspondingly performed.

Based on self-reported data on weight and height we calculated the body mass index (BMI). Overweight was defined as a BMI of 25 kg/m2 or more whereas obesity was defined as a BMI of 30 kg/m2 or more.

Current smoking of cigarettes, cigars, cigarillos or pipes or cessation of smoking within the last two years was classified as current smoking. Stopping smoking more than two years ago was classified as former smoking. Never having smoked regularly for at least 1 year was classified as never-smoking. Daily use of moist snuff/Swedish tobacco in the preceding year was classified as use of smokeless tobacco (as opposed to non-use).

Physical inactivity was defined based on reports about level of leisure time physical activity in the previous 5–10 years. Reports of “very little exercise” or “isolated walks only” were considered exposed. The reference category includes “regular exercise (at least once weekly)” and “exercise once in a while”.

The sitting time was asked in relation to daily working hours in the previous 5–10 years. Reports of “almost all the working hours” were considered exposed. Reports of “half of the working hours” or “less than half of the working hours” were considered unexposed.

Previous non-MI CVD was defined as the presence of any of the following conditions: heart failure, stroke, angina and intermittent claudication.

Based on answers to questions with predefined answer alternatives about diagnoses and causes of death (if relevant) in parents and siblings before they turned 65 years old, participants were classified into categories of maternal, paternal and sibling history of CHD and CVD respectively. CHD comprises sudden death, MI and angina whereas CVD comprises CHD and stroke. “Don’t know” was also included as a predefined answer alternative.

Our analyses used different approaches: A) Considering individuals who provided full information about history of disease and cause of death (if relevant) in the parent and potential siblings, while excluding individuals reporting “don’t know” to either of these questions. B) Considering individuals who provided any information about history of disease or cause of death (if relevant) in the parent or potential siblings. In approach B, “don’t know” answers were set to “no”.

Dietary habits, intake of alcohol, coffee and vitamin supplements. The participants were asked to report the average daily or weekly intake of the following dietary items in the preceding year: 1) Fruit and berries, 2) Roots and vegetables (except potatoes), 3) Meat and sausage dishes, 4) Fish dishes and 5) Use of shortening, cooking oil and sauce, respectively. The number of servings per occasion was asked for as well as how often the dish was on the menu. In order to have the total number of servings per week we multiplied the number of times per week reported by the number of daily servings reported for each dietary item. We considered as exposed those who reported frequency of intake above the median level of intake in the group of control individuals. The following cut-off levels were used: 1) Fish, one serving per week; 2) Use of shortening, cooking oil and sauce, 4 servings per week; 3) Meat and sausages, 3 servings per week; 4) Roots and vegetables, 6 servings per week and 5) Fruits and berries one serving daily. Study participants who reported coffee drinking were asked how many cups consumed either weekly or daily in the preceding year. We calculated the number of cups daily and chose a cut-off of 3 cups daily (based on the median consumption of coffee among controls) for exposure to high coffee consumption. The intake of less than 3 cups daily was used as reference category. The regular intake of vitamin supplements: vitamin supplements, minerals or other dietary supplements were assessed simply from answers to a yes/no question regarding intake in the preceding year. For the present study, we consider information about frequency of intake of light beer, strong beer, wine and hard liquor in the preceding year as well as in the preceding 5–10 years. In addition, the serving sizes of each alcohol beverage (number of cans, glasses or bottles consumed at each drinking occasion) were considered. For the reporting of frequency of light and strong beer intake in the preceding 5–10 years, four pre-defined categories were given: 1) Never, 2) One or two cans/bottles per week, 3) Three to nine cans/bottles per week and 4) Ten cans/bottles per week or more. For the reporting of wine and hard liquor consumption in the preceding 5–10 years, the predefined categories were: 1) Never, 2) Once monthly, 3) Every week and 4) Every day. For classifying individuals as exposed to high intake of each specific beverage, we used as cut-offs the median frequency of intake in the distribution in controls. The cut-off for both light and strong beer was 3 cans/bottles per week or more. For both wine and hard liquor the cut-off was “Drinking every week or more often”. Regarding serving sizes of wine and hard liquor in the preceding 5–10 years, the following predefined answer alternatives were given: 1) More than one glass, 2) A couple of glasses, 3) Half a bottle and 4) One bottle or more. The median values of the controls´ distributions were used as cut-off. Participants classified as exposed to large serving size of wine and hard liquor in the preceding 5–10 years, respectively, were those who reported a serving size of half a bottle or more. Based on reports about frequency of intake of each of the alcohol beverages in the preceding year, as well as information about serving sizes, we calculated the average daily alcohol consumption in grams. Participants with values above the median value of the distribution in controls (10.02g) were considered exposed to high alcohol consumption. Participants with values of 10.02g or lower were considered unexposed. The latter group includes participants reporting no alcohol intake.

Work-related factors. Job strain was determined using the Swedish version of the demand-decision latitude questionnaire implemented in SHEEP [10]. The subject reported the average situation in the preceding 5 years. The demand sum score, the decision latitude sum score and their ratio were calculated for each subject. The 75th percentile of this ratio amongst all the controls in the SHEEP study (0.765) was used as the cut-off value to define exposure to job strain according to the “Quotient job strain model”; subjects with a score above the cut-off value were classified as exposed to job strain and all other respondents were considered unexposed. Binary variables were created for having subordinates, working shifts, receiving a monthly salary and having previously been unemployed. Exposures to different pollutants at workplace: motor vehicle gases, particulate matter, combustion by-products, lead and dynamite were assessed using 3 possible answers: “yes”, “no” and “don’t know”. Those who reported presence of any of the pollutants were considered exposed to pollution at workplace. Those who answered “no” to all pollutant items were considered unexposed, whereas individuals answering “don’t know” were excluded from the analysis. Psychosocial factors. The economic situation of the family during childhood was assessed from questions about economic problems before the age of 16. Three predefined answer alternatives were given: 1) No economic problems important enough to mention, 2) Small and/or reasonably short-term economic problems, and 3) Severe and/or long-term economic problems. We created a variable called “Economic problems before age 16” where answer alternatives 2 and 3 were considered exposed. A variable called “Severe economic problems before age 16” was also created, where answer alternative 3 was considered exposed whereas answer alternative 1 and 2 were considered unexposed. Based on the highest educational level reported by the participants, three categories were formed: 1) Compulsory education (9 year of education), 2) Complete high school (12 years of education) and 3) University (more than 14 years of education). For the analysis of compulsory education we considered as exposed those belonging to that category; other responses were considered unexposed. In the same fashion, for the university education we considered as exposed those belonging to that category; other responses were considered unexposed. For the following stressful events, if they occurred within a year before the survey, binary variables were created: 1) Conflict with spouse, 2) Death of relative or friend, 3) Disease/accident in spouse, 4) Death of a close relative/friend, 5) Impaired personal finances, 6) Conflict at workplace, 7) Moving, 8) Change of job, 9) Decreased responsibility at work and 10) Increased responsibility at work. Four questions related to coping strategies were posed to participants: 1) “How often did you feel you could not control important matters in your life?”, 2) “How often did you feel confident in your ability to handle your personal problems?”, 3) “How often did you feel things turn out the way you wanted?” and 4) “How often did you feel you could not manage the difficulties?” For each item, there were five possible responses: 1) “never”, 2) “almost never”, 3) “sometimes”, 4) “often” and 5) “very often”. We dichotomized those ordinal variables according to the median among controls: We considered as exposed those who answered “often” or “very often” in the questions 1 and 4. For questions 2 and 3 we considered exposed those who answered “sometimes”, “often” or “very often”. In all the four items, all who answered any other response were considered unexposed.