Study population
The study population is from the RHINESSA generation study - Respiratory Health in Northern Europe, Spain and Australia (www.rhinessa.net). One parent of each offspring in RHINESSA had previously participated in the Respiratory Health in Northern Europe (RHINE) study (www.rhine.nu), part of the European Community Respiratory Health Survey (ECRHS) initiated in 1989–1992 with three study waves (www.ecrhs.org) performed ten years apart. RHINE data was collected from seven different centres in five countries: Reykjavik (Iceland), Bergen (Norway), Umeå, Uppsala and Göteborg (Sweden), Aarhus (Denmark) and Tartu (Estonia). Furthermore, offspring with parents from three additional ECRHS centres, namely Melbourne (Australia), and Huelva and Albacete (Spain), were included in the RHINESSA population. Written consent was obtained from each participant in all study centres. In total, 8260 offspring from the RHINESSA cohort were matched with one of their parents (n = 6045). We used self-reported questionnaire information about smoking status from the parents themselves (collected in RHINE III/ECRHS III (2010–2012) or RHINE II/ECRHS II (1999–2000)) as well as offspring-reported parental smoking status (collected, among adult offspring (> 18 years), 2013–2016). Smoking during pregnancy was assessed from a specific women’s questionnaire in the RHINE III study. The questionnaires used have been developed in the framework of ECRHS, RHINE and RHINESSA and are used in several studies before. All questionnaires are available online from the study webpages listed above.
We used information from 7185 adult offspring (18-51 years) with information on one of their parents, n = 5307 (27-67 years), obtained in prioritised order from either RHINE III, ECRHS III, RHINE II or ECRHS II with full information as the eligibility criterion. In the centres of Melbourne, Huelva and Albacete, all smoking data were assessed by interview-administered questionnaires while data from all other centres were mostly assessed by a self-administrated questionnaire.
A flowchart of the study population is provided in Fig. 1. Additionally, in the analysis of smoking during pregnancy, we used data from women participating in Rhine III, who also answered the questionnaire on women’s health, 807 offspring and their mothers (n = 679).
Reporting of smoking status
Parental smoking status during offspring’s childhood was defined based on the parents’ date of birth, age of smoking debut or current age and number of years they had smoked (current smokers) or the year they had quit (ex-smokers). Offspring’s childhood was defined as the age period 0–10 years.
Parents’ own report of smoking status
Parents’ report of their own smoking status during their offspring’s childhood was accessed slightly differently, do to slightly different questions, in the different surveys and study waves. The questions used were:
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A.
RHINE III: “Do you smoke? (this applies even if you only smoke the odd cigarette/cigar or pipe every week) (No/Yes)”, “Did you smoke previously? (No/Yes)”, “How old were you when you started smoking? (...years)”, “For how long have you smoked? (applies to both smokers and ex-smokers) (...years)”, and “If you are an ex-smoker, when did you stop smoking? Year ...”
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B.
RHINE II: “Are you a smoker (this applies even if you only smoke the odd cigarette/cigar or pipe every week)? (No/Yes)”, “Are you an ex-smoker? (No/Yes)”, “Smoked for …years (applies to both smokers and ex-smokers)”, and “Stopped smoking in … (year)”
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C.
ECRHS III and ECRHS II: “Have you ever smoked for as long as a year? [‘YES’ means at least 20 packs of cigarettes or 12 oz. (360 grams) of tobacco in a lifetime, or at least one cigarette per day or one cigar a week for one year] (No/Yes)”, “How old were you when you started smoking? (Years)”, “Do you now smoke, as one month ago? (No/Yes)”, “Have you stopped or cut down smoking? (No/Yes)”, and “How old were you when you stopped or cut down smoking? (Years)”
Mothers’ own report of smoking status during pregnancy
In the “RHINE III – Women’s questionnaire”, mothers’ report of own smoking status during pregnancy was identified through the question: “During this pregnancy (tick if yes)… Did you smoke?” The mothers’ report of own smoking status during pregnancy was dichotomized according to whether or not they smoked, and correct pregnancy (in case of siblings) was identified through offspring’s birth year.
Offspring-reported parental smoking status
In the RHINESSA study, offspring-reported parental smoking status during the offspring’s childhood was obtained through the questions “Did your father ever smoke regularly during your childhood? (No/Yes/Don’t know)”, “Did your mother ever smoke regularly during your childhood? (No/Yes/Don’t know)”. Their mothers’ smoking status during pregnancy, on the other hand, was obtained through the question “Did your mother smoke when she was pregnant with you? (No/Yes/Don’t know)”. Offspring-reported parental smoking status during the offspring’s childhood and offspring-reported maternal smoking status during pregnancy was dichotomized as “Yes” or “No” and the “Don’t know” category was treated as missing data. For 2.3% of the fathers and 1.4% of the mothers, their offspring answered “Don’t know” to the question on whether they had smoked during the offspring’s childhood, while 23.1% of the mothers’ smoking status during pregnancy was categorized as “Don’t know” by the offspring.
Predictors for disagreement
Offspring’s sex, age, educational level, asthma status (self-reported doctor diagnosed asthma), own smoking status and own parental status were included as potential predictors for disagreement. Offspring’s sex was included in the model to examine sex differences in reporting of their parent’s smoking; age was included to examine the differences in time trends and risk of recall bias as a predictor for discrepant answers. Educational level was included to examine if the well-studied “health-education gradient” would be a predictor for awareness of parents’ health behaviour. Offspring own asthma status was included as we hypothesise children’s own asthma status will influence their recall of their parents smoking. Offspring’s own smoking status and parental status were included to examine if their own behaviour were increasing their awareness about their parents’ behaviour. Asthma, smoking status and own parental status were dichotomized as “No” or “Yes”. Offspring’s age was categorized as “< 25 years”, “≥25 < 35 years” or “≥35 years” (reference). Education was categorised as “primary school” (reference), “secondary school/technical school”, or “college or university”. Parents’ sex and average amount of smoking were also included in the analyses as predictors for disagreement. Parents’ sex was included in the model to examine any sex differences, where amount of smoking was included to examine if higher amount of smoking had increased the awareness among the offspring. Parents’ amount of smoking was obtained in the surveys based on the following questions:
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A.
RHINE III: “How much do you smoke / did you smoke? (give an average …cigarettes/day,…cigars/week, …pkts pipe tobacco/week”
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B.
RHINE II: “Smoke/smoked …cigarettes/week, …cigars/week, …pkts pipe tobacco/week”
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C.
ECRHS III and II: “On average of the entire time you smoked, before you stopped or cut down, how much did you smoke? Number of cigarettes per day…, number of cigarillos per day…, numbers of cigars a week…, pipe tobacco in b) grams/week…”.
The different tobacco products were converted to cigarettes. One cigarillo was converted to three cigarettes, one cigar to five cigarettes, and one gram pipe tobacco equalled one cigarette [12]. The amount of smoking was dichotomized as 10 or more cigarettes per day or less than 10 cigarettes per day on average.
Statistical procedures
Sensitivity and specificity were calculated with 95% confidence intervals [CIs]. Cohen’s Kappa estimate [K], with 95% CIs, was calculated to estimate the agreement of the offspring-reported parental smoking status during childhood and pregnancy with the parent reported smoking status during the same periods. Parents’ own report was considered the gold standard. Cohen’s Kappa interpretation was based on the following categories: poor agreement, < 0.2; fair, 0.21–0.40; moderate, 0.41–0.60; good, 0.61–0.80; and very good, 0.81–1.00 [13]
Multivariate logistic regression models were conducted to estimate the odds ratio [OR] of whether offspring’s sex, age, educational level, asthma status, own smoking status or own parental status and parents’ sex and amount of smoking were predictors for disagreement. The regression models were performed with repeated measurements due to multiple offspring from the same parent using proc. GENMOD function in SAS. The model was mutually adjusted for the included variables and further adjusted for study centre. Disagreement was defined as discrepant answers between offspring and their parents. The significance level was set at a p-value of < 0.05 (two-sided) and 95% CIs were calculated.
We performed sensitivity analyses where those excluded due to missing co-variables were included in the calculation of sensitivity, specificity and K. Furthermore, we performed sensitivity analyses where childhood was expanded to cover 0–18 years. We performed analysis by sex of the parents, and by the offspring’s own parental status. Subgroup analyses were conducted to examine if parents’ amount of smoking affected the sensitivity, specificity, or agreement. Further subgroup analyses were performed to examine whether false answers from the main analyses were false positives or false negatives. Additionally, sensitivity analysis with a reversed priority line (1. RHINE II, 2. ECRHS II, 3. RHINE III, and 4. ECRHS III) of the included data from the parents was performed to examine if the period of the data collection and perception of smoking at that time had an impact on the estimates. Separate analyses of centres were conducted for sensitivity, specificity and K, and adjustment for study centre was applied in the logistic regression models. All statistical analyses were performed using SAS (SAS Institute Inc., Cary, NC, USA) version 14.1.