Across advanced economies, technological change is continuously reshaping occupational task structures, increasing the need for skill updating beyond initial education and within ongoing employment relationships. In response, workers predominantly rely on short-format, job-specific forms of non-formal further education, delivered both in face-to-face settings and, especially since the COVID-19 pandemic, through online formats. In policy debates on lifelong learning, micro-credentials have therefore emerged as a key instrument to enhance the transparency, portability, and recognition of these learning activities (European Commission, 2021). However, participation in non-formal further education and the acquisition of micro-credentials are unequally distributed and strongly structured by occupational and organisational contexts. Moreover, the labour market returns to such training, measured in income gains or improved employment prospects, vary substantially across these same structures (Ehlert, 2017; Haupt, 2012).

One influential explanation for these patterns is derived from social closure theory (Bills, 2003; Bol & Weeden, 2015; Weber, 1925; Weeden, 2002). From this perspective, occupations differ in the extent to which access is regulated through formal credentials. In occupationally closed labour markets, entry is restricted by legally or institutionally required certificates, whereas in open occupations formal entry barriers are weak or absent. Where access is tightly regulated, formal qualifications should already provide a sufficient and credible signal of occupational competence. Consequently, additional non-formal training certificates are expected to yield lower (or even no) returns, as they convey limited incremental information to employers (Haupt, 2012). In contrast, in open occupations, where formal credentials provide weaker signals, additional certificates may play a more substantial role in recruitment decisions.

Empirical studies have provided support for these theoretical expectations, for example by demonstrating heterogeneous income returns to further education across occupational segments (Ehlert, 2017). However, direct tests of closure-theoretical predictions at the level of hiring decisions remain scarce. In particular, little is known about how employers evaluate non-formal training certificates during recruitment (Schimke, 2023) and whether their signalling value systematically differs between open and closed occupational labour markets. This gap is particularly salient in light of the growing policy emphasis on lifelong learning and the proliferation of short-cycle and micro credentials (European Commission, 2021).

This paper addresses this research gap by examining whether occupational closure moderates the signalling value of non-formal training certificates in hiring. The central research question is: Do structural differences in the signal value of further training certificates emerge between occupationally open and closed labour markets? Building on social closure theory and signalling approaches, we argue that the evaluation of additional certificates depends not only on their content but also on the institutional context of the target occupation.

By focusing explicitly on employers’ hiring decisions, this study contributes to the literature in three ways. First, it provides a direct test of closure-theoretical assumptions in a recruitment context. Second, it offers experimental evidence on how different types of non-formal credentials are valued relative to formal education and work experience. Third, it sheds light on the conditions under which micro credentials may enhance employability. The analysis is based on a preregistered experimental study (https://osf.io/5z4ky) conducted among recruiters in Germany, covering a broad range of occupational fields with varying degrees of formal regulation.

The empirical analysis is based on a preregistered discrete choice experiment conducted in February and March 2025. The study population consists of personnel recruiters who make hiring decisions in at least one of 24 systematically selected occupations. These occupations were chosen to represent (1) fields in which a higher education degree is typically required, (2) key sectors of the German labour market, and (3) an equal number of occupationally closed (regulated) and open (unregulated) professions, resulting in 12 occupations in each category. Recruiters were identified with a stratified random sampling strategy and contacted using contact information from job advertisements they had placed on the website of the Federal Employment Agency between October 2024 and January 2025. In total, 1,380 respondents participated in the survey (response rate: 16%). Each respondent completed three independent hiring scenarios. In each scenario, they were presented with three applicant profiles displayed in a tabular CV format and asked to select the candidate they would hire, with an additional opt-out option. Applicant profiles varied systematically along five core dimensions: (1) formal educational degree (Bachelor’s vs Master’s), (2) match between field of study and target occupation (exact vs related), (3) length of work experience (1, 3, or 5 years), (4) occupational match of work experience (exact vs related), and (5) micro credentials (none, face-to-face, online). For profiles including a micro credential, three additional attributes were experimentally varied: (6) training provider (university vs private company), (7) course duration (40 vs 80 hours), and (8) type of certification (attendance vs examination). The full vignette universe comprised 576 unique profiles, which were combined into choice sets of three profiles. Using the idefix package in R (Traets et al., 2020), the design was reduced to 162 efficient choice sets while maintaining near-orthogonality, balanced level distributions, and minimal overlap (Auspurg & Liebe, 2011; Huber & Zwerina, 1996). These sets were divided into 54 blocks, with each respondent randomly assigned to one block. The order of choice sets and the position of profiles within sets were fully randomised. The resulting data include 4,016 hiring decisions and evaluations of 12,048 applicant profiles. Effects of micro credentials on hiring probabilities were estimated using conditional logit models. Substantive interpretation is based on average marginal effects (Breen et al., 2018; Chamberlain, 1980).

The results provide nuanced insights into the role of micro-credentials as signals in personnel recruitment. Overall, certificates obtained through face-to-face programmes are associated with a significantly higher probability of being selected, whereas online micro credentials are, on average, evaluated no differently from profiles without additional certificates. This finding suggests that recruiters distinguish sharply between different forms of micro-credentials. With respect to social closure theory, the findings are both confirming and challenging. Contrary to theoretical expectations, the positive effect of certificates from face-to-face courses does not differ significantly between occupationally open and closed professions. This indicates that such certificates may convey valuable information beyond formal entry requirements even in regulated occupations. In contrast, clear differences emerge for online micro-credentials. In occupationally open professions, these certificates are associated with a positive and significant increase in hiring probabilities. In closed occupations, however, they are evaluated negatively relative to having no additional certificate at all. These results suggest that the signalling value of online credentials is highly context-dependent. In regulated professions, they may be interpreted as weak, redundant, or even inappropriate signals that conflict with established credential hierarchies. In open occupations, by contrast, they appear to function as credible indicators of motivation and/or skill acquisition. The findings contribute to current policy debates on lifelong learning, corporate training strategies, and skilled labour development. They underline that expanding access to online credentials alone is insufficient and that their labour market value crucially depends on occupational institutions and recruitment practices.

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