Most of the quantitative literature published on green public procurement (GPP) is based on surveys with low response rates, which may lead to respondent bias. The authors surveyed the largest contracting authorities in the Czech Republic and the Slovak Republic, analysing thousands of contract notices. With a 43.2% response rate, it was found that authorities not participating in the GPP survey tended to adopt less GPP due to reasons such as lack of prior experience, fear of negative publicity, or insufficient permissions to share information. The authors’ findings cast doubt on the validity of studies with low response rates or those that do not rely on experimental design, as they may lead to an under-representation of GPP non-adopters.