Data from the KiGGS used in this investigation are based on self-administered questionnaires completed by parents/caregivers, computer assisted personal interviews (CAPI) with parents/caregivers and children/adolescents’ self-reports of HRQoL. The CAPI was conducted by physicians who were specifically trained for the purposes of the study and, besides vaccination status and medication use within the past 7 days, assessed the history of selected physician-diagnosed conditions. If parents gave their consent, a blood sample was taken. Using the system IMMUNOCAP (Phadia), specific IgE was determined in those children aged 3–17 years for the following antigenes: Dermatophagoides pteronyssinus, Dermatophagoides farinae, cat dander, horse epithelium, dog dander, egg white, peanut, soy bean, milk protein, carrot, potato, wheat flour, green apple, rice, rye pollen, timothy grass pollen, Cladosporium herbarum, Aspergillus fumigatus, birch pollen, mugwort pollen .
The 4-week and the 12-month prevalence of eczema, asthma and hay fever/allergic conjunctivitis were based on parents’ reports of a medical diagnosis made by a physician within the past four weeks or 12 months, respectively in the CAPI.
HRQoL was measured by the German KINDL-R questionnaire which contains 24 items . Its psychometric properties have been established . Generic in nature it measures the following six dimensions of quality of life: Physical well-being (PWB), emotional well-being (EWB), self-esteem (SE), family (quality of relationship with family), friends (quality of relationship with friends), and everyday-functioning (well-being at school). The recall period is the past 7 days. It accounts for differences in development by providing different versions for different age groups. Items are scored on a 5-point Likert scales (never, seldom, sometimes, often, always). For each dimensions a total score can be computed by adding the respective items forming that dimension. An overall total KINDL-R score is computed by summing all 24 items. All scores are transformed into values ranging from 0 – 100. Larger scores denote better quality of life. For the present analysis the self-report version of the KINDL-R was used.
The database used for the present analyses further contains information (assessed by parent report) on age in years, gender (male vs. female), migration status (German versus migration background), weight and height. The latter two were used to compute the Body-Mass-Index which was used to classify overweight according to Krohmeyer-Hausschild . It further contains data (education, occupation and family income) which is used to create the composite socioeconomic status (SES; low, medium, high) . Finally, allergic sensitisation was operationalised as yes (sensitization to at least one of the above food or aero-allergens) or no. All analyses were adjusted for these variables. Covariates were chosen based on evidence regarding their relationship with HRQoL and/or the atopic conditions.
The self-report version of the Strengths and Difficulties Questionnaire (SDQ) , a brief 25-item behavioural screening questionnaire assessing emotional symptoms, conduct problems, hyperactivity/inattention, peer relationship problems and prosocial behaviour was used to provide a measure of mental health (normal, borderline, abnormal). Individual SDQ-items are scored on a 3-point scale (0 not true, 1 somewhat true, and 2 certainly true); higher scores denote greater problems except for prosocial behaviour, for which a higher score is suggestive of more positive behaviour. All items apart from the prosocial behaviour items are used to calculate a total score (range 0 – 40) with higher scores indicating more problematic behaviour. According to the cut-offs provided  individuals are assigned to one of the three groups (≤ 15 = normal, > 15 = borderline, > 19 = abnormal mental health). Although the self-report SDQ’s reliability and validity have been established across cultures to some extent , it has to be noted that the SDQ fares less well than a full clinical interview in terms of validity .