Barrett, A., Hovels, B., and Kraayvanger, G. (1998). Exploring the Returns to Continuing Vocational Training in Enterprises: A review of research within and outside of the European Union, Thessaloniki: CEDEFOP, https://www.esri.ie/publications/exploring-the-returns-to-continuing-vocational-training-in-enterprises-a-review-of-research-within-and-outside-of-the-european-union