Adolescent survey non-response and later risk of death. A prospective cohort study of 78 609 persons with 11-year follow-up

Non-response has become a growing concern in survey studies. Consequently, extra effort is required to determine its effect on study results. In cross-sectional designs, non-respondents are selected, for example, by poorer health behaviour, health and socio-economic background [1, 2]. In cohort studies, health outcomes during follow-up may be related to baseline non-response [3], leading to under- or overestimation of the outcome. Only few studies, with controversial results, have described the association between non-response and death rates [4–6].

No information exists about adolescent non-respondents and their later risk of death from injury and other causes. In Finland, we had the unique opportunity to link a large representative adolescent cohort, including both survey respondents and non-respondents, to national death registry in order to see whether there is a risk of death, and which type of death, related to non-response at baseline.

The aim of this prospective cohort study was to investigate whether adolescent survey non-respondents are selected according to later risk of injury- and disease-related death.

When the baseline cohort was analysed separately by sex, we noted that non-response in the entire female cohort was associated with an increased risk for all causes of death in the follow-up (2.2 times, 95% CI: 1.5–3.2), while the corresponding risk among non-respondent males was 1.8 (95% CI: 1.5–2.2) (Table 3.). The associative effect of non-response on death increased with age at baseline. Non-response among 16 to 18-year-old males was associated with death caused by disease (HR 2.6; 95% CI: 1.9–4.3), intoxication (HR 2.6; 95% CI: 1.5–5.6) and unintentional injury (HR 1.9; 95% CI: 1.3–2.8). When the female cohort was analysed by age at baseline, non-response was associated with disease-related death at ages 16 to 18 (HR 4.1; 95% CI: 2.3–7.3). (Table 3.)

Our findings demonstrated that adolescent survey non-respondents are selected according to later risk of death. The risk was higher among non-respondents compared to respondents. The excess mortality was particularly associated with disease-related and intoxication deaths, while the difference was smaller for deaths by unintentional and violence-related injury. In this follow-up from adolescence into adulthood, non-respondent males had a significantly increased risk of death by disease, intoxication, violence-related injury and unintentional injury, whereas non-respondent females showed an increased risk of death by disease only. Moreover, the increased risk persisted beyond the age of 25. The association between non-response and risk of death persisted with age better in males than in females.

This study has notable strengths. First, we had the exceptional opportunity to measure the outcome independently of the survey. Second, the study was based on a large, prospective, nationwide sample of adolescents followed over a remarkably long period (876 400 person-years). Third, the coverage and accuracy of the Finnish Cause-of-Death Statistics are known to be excellent [7–9].

The death rates varied between respondents and non-respondents by the type of death. Only few reports describing death rates among non-respondents exist [3, 6], but none of them has focused on adolescents. In a study on Taiwanese adults, persons not responding to health surveys had more deaths in a follow-up of two years [4]. A similar finding was published in the United Kingdom, where men not participating in a cardiovascular disease study showed an increased risk of death compared to men participating for a period of three years, after which the difference between the groups disappeared [5]. However, their risk of cardiovascular death did not differ. It should be noted that the follow-up periods of only a few years can be considered relatively short and thus a limitation in these studies.

The degree of a possible bias effect of non-response on adolescent injury and violence survey results has not been previously investigated. Our study showed that, in the follow-up, unintentional injury death rates were 1.8 times, violence-related injury deaths 2.0 and intoxication deaths 3.2 times higher among adolescent non-respondents compared to respondents. The increased risk of injury was especially seen in males. It is evident that non-respondents sustain more injuries, intoxications and violence than respondents. Furthermore, it is remarkable that the death risk associated with non-response persisted after adolescence, even beyond the age of 25. Based on our findings, it seems obvious that the occurrence of injuries and violent deaths as based on cohorts in a survey setting is a clear underestimation.

A previous Finnish study showed that survey non-respondents tend to engage in negative health behaviours, such as smoking, as well as suffer from mental disorders more often than respondents [1]. Moreover, it seems plausible that several risk factors for injuries accumulate in the non-respondent group. Strong evidence exists that use of alcohol, smoking, poor health and low sociodemographic status predict injuries in adolescence [10]. It seems that non-response, particularly in males, is not distributed sporadically but may be regarded as an indicator of a health neglecting lifestyle and a predictor of injury-related death.

Interestingly, non-response was associated with an increased risk for disease death, even more than injury death. Non-response in disease deaths may be a partial consequence of an already existing serious disease or disability at the time of survey. Unfortunately, we had no opportunity to measure this. The survey sample was drawn from the National Population Register Centre through the selection of all Finns born on certain days. In the follow-up, we used the cause-of-death statistics from the survey endpoint, the 30^th of April of the survey year and thus, even if death had occurred between the selection of the sample and the response time, it was not registered as death in our analysis.

In our sample, non-response was more common in males and in the older age groups. For example, among 18-year-old males, the response rate was relatively low (67%). A more specific analysis illustrated that while the response rate was 67% in the 18-year-old males and 85% in the 18-year-old females, the injury risk in the non-response group was elevated only in the non-respondent males. In males, the declining response rates from the 1980s to the 1990s did not affect the association between response status and deaths. In females, however, the declining response rates slightly decreased this association.

In summary, the present prospective cohort study is the first to explore death risk among adolescent survey non-respondents. Although death rates vary by death type between respondents and non-respondents, this study shows that the risk of death from any cause (intoxication, violence-related injury, unintentional injury or disease) is increased among adolescent non-respondents and the increased risk persists after the age of 25 years. The predictive strength of non-response, however, seems to persist with age better in females than in males.