The main threat to the external validity of survey results is attracting a large enough and representative sample of the population, which is generally difficult to obtain for studies
on professional software engineers. We used outsourcing on freelance marketplaces to overcome this challenge. We refer to our companion paper for a detailed discussion of the challenges and opportunities of this approach [37], but repeat here that the challenges lie in uncertainty about background and skills of participants, and it is difficult to objectively assess aspects such as competence, education and experience.