  In this study we used company level administrative data from the National
Labour Inspectorate and The Polish Social Insurance Institution in order to
estimate the prevalence of informal employment in Poland. Since the selection
mechanism is non-ignorable we employed a generalization of Heckman's sample
selection model assuming non-Gaussian correlation of errors and clustering by
incorporation of random effects. We found that 5.7% (4.6%, 7.1%; 95% CI) of
registered enterprises in Poland, to some extent, take advantage of the
informal labour force. Our study exemplifies a new approach to measuring
informal employment, which can be implemented in other countries. It also
contributes to the existing literature by providing, to the best of our
knowledge, the first estimates of informal employment at the level of companies
based solely on administrative data.
