Sense of coherence and attrition during four-year follow-up in cohorts of permanent and non-permanent Finnish employees

The epidemiological knowledge of work-related risks is based mostly on follow-up studies of occupational cohorts in particular workplaces. Such studies are becoming more complicated, as the instability of work life is increasing due to increasing proportions of various temporary job contracts and higher employee turnover between organisations and occupations. In addition to focusing on the cohort that remains in stable employment during the follow up, epidemiological studies should also trace those employees who left their workplaces, and control for bias related to such attrition.

Sample attrition and the accompanying bias impair the interpretation of the results of health surveys. The reasons for attrition are, in addition to refusal to participate, death or severe illness and failure to locate the individuals. Distinguishing those who cannot be contacted is important, because the mechanism of non-participation may influence the nature of the attrition bias [1–3]. In a cohort study within a workplace or organisation one notable reason for attrition is turnover of workers, which includes many sub-categories (e.g. transfer to other workplaces, layoffs and outsourcings, retirement). Instead, in a study of the cohort also including employees who leave the organisation, difficulties in finding individuals' place of residence and address are important reason for attrition.

To the best of our knowledge, only a few longitudinal population-based studies have taken into account sample attrition related to the working situation. The employed tend to participate more actively than unemployed [1, 4, 5]. Badawi et al. [6] found that the unemployed were more difficult to trace and died more often, but unemployment did not influence refusal to participate. The traditional dichotomy of unemployed versus employed, however, is too crude for assessing the modern labour market, where employees may occupy several statuses between permanent jobs and overt unemployment. For example, there is no research among fixed-term employees.

The existing research on the determinants of attrition focuses on sociodemographic factors and health, including mental disorders, but psychological characteristics that may also predict sample attrition are rarely included in these studies. We were interested in the sense of coherence (SOC) [7, 8], as a possible source of bias in a cohort study. According to Antonovsky's theory of SOC, a person with strong SOC stays healthy despite possible stressful encounters and a number of empirical studies show that strong SOC is indeed associated with various aspects of good health [9–13]. SOC-related non-response might be hypothesised to be as a potential source of bias in health-related survey studies, but there is no research available on a possible association of SOC with attrition or participation.

The aim of this study was to examine sample attrition in a four-year follow-up study among cohorts with initially permanent and non-permanent employment status. We examined baseline characteristics between employees who participated in a follow-up study compared with those who did not. Non-participation was categorised into drop-out due to loss and drop-out due to refusal. Our particular aim was to investigate SOC as a possible source of bias, i.e. we asked whether low SOC predicts refusal to participate in a follow-up survey or exit from the work place or disappearance from the population register. Moreover, we aimed to investigate the associations of participation with sociodemographic characteristics and health in the cohorts of permanent and non-permanent employees.

This study examined sample attrition in cohorts of permanent and non-permanent employees. The focus was on the associations of baseline SOC with non-response to follow-up survey. In addition, we examined whether SOC predicted contact failure due to exit from the work place among permanent employees and contact failure due to unlocated address among fixed-term and subsidised employees. Corresponding associations with sociodemographic factors and self-rated health were also studied.

We found that low SOC predicted low response rate in the cohort of fixed-term employees. Contact failure was not related to SOC among permanent employees, i.e. those who left work for "natural" reasons such as a move to another workplace, retirement or death, had only slightly lower SOC than the respondents. Among non-permanent employees contact failure tended to be more common in those with low SOC, but this association did not reach statistical significance.

According to Antonovsky [7, 8] SOC predicts and explains an individual's movement along the health-disease continuum. He viewed SOC as individuals' global orientation or disposition leading to a certain attitude towards their internal and external environment [8]. A strong SOC means good resources to resist stressful encounters, including work-related stress and other exposures. In line with this, Feldt [16] found that employees with strong SOC had fewer psychosomatic symptoms and lower level of emotional exhaustion at work. Antonovsky's theory also suggests that SOC stabilises by the age of about 30, and the first years of employment are important in this development [8]. The period during which individuals leave their studies to enter the labour market appears particularly important [17]; if the early career is characterised by insecure entry into work, SOC may never reach the optimal level.

In our study, the permanent employees were older than the fixed-term and subsidised employees, and it is evident that majority of them should have had a stable SOC. However, as most of the non-permanent employees were older than 30, and we may assume that their SOC was also relatively stable.

Research in the field of psychiatry has documented that psychiatric disorders and psychopathology increase sample attrition due to loss of contact rather than refusal [6, 18]. According to our results SOC does not follow this tendency. In other words it seems to be inappropriate to consider SOC simply as an indicator of mental morbidity or psychological well-being.

Poor self-rated health has been shown to predict sample attrition [1, 5, 19]. Our findings confirm this in the cohort of permanent employees, both with regard to non-response and to exit. In fixed-term employees the point estimate showed an opposite though statistically non-significant association with regard to non-response. On the other hand, among them responding was associated with high SOC. In all, these findings add one more paradox to the body of research concerning health and well-being of fixed-term employees [see e.g [20]].

Women tend to participate in follow-up more actively than men [19, 21–23], although there are some conflicting findings [2, 24]. In this study women also were more likely to respond in all cohorts. Several studies have explained this by pointing to gender differences in health. However, in our study such an explanation was unlikely because the analyses were adjusted for self-rated health at baseline.

The probability of responding is reported in most studies to increase with age [7, 10, 12], although there are studies of elderly cohorts reporting opposite findings [3, 12]. In our study the cohorts were relatively young. Thus, our finding of less attrition among older participants was to be expected.

Higher response rates have been reported among those with a higher socio-economic status [1–3, 6, 18, 19, 21–24]. This finding was replicated in our study but only among permanent employees.

This study, concentrating on associations between SOC and sample attrition during follow-up, does not permit conclusions of a potential SOC-related participation bias in the initial survey. It is possible that the effect of SOC is different in surveys conducted among unselected populations.

In the past, it was easier to follow up occupational cohorts throughout their working careers, as it was common for individuals to stay in one workplace. Nowadays such follow-up is difficult due to greater turnover in occupations and employers and employees. This study showed that it is possible to trace over 90 per cent of non-permanent employees by utilizing the Finnish Population Register. Inclusion of non-permanent employees, however requires, special arrangements, and among them sample attrition may be related to different factors than among permanent employees. Our finding of non-random sample attrition in relation to SOC serves as an example of factors which must be considered when following up occupational cohorts that include fixed-term employees.