NBER working papers are generally considered reliable sources for academic research, including thesis work, despite not being peer-reviewed in the traditional sense. The National Bureau of Economic Research (NBER) is a well-respected institution known for its rigorous research standards. These working papers often contain preliminary findings or ongoing research by established economists, which means they can offer insights into cutting-edge developments in the field before they are formally published in peer-reviewed journals. However, their preliminary nature means that the research might undergo further refinements, and the conclusions might not be final. As such, when citing NBER working papers in a thesis, it's advisable to treat them with caution, acknowledging their status as pre-publication work. This involves critically evaluating the methodology, data, and conclusions, and possibly supplementing with peer-reviewed sources where possible to validate or contrast the findings. It's also good practice to note the date of the working paper to account for any updates or revisions that might have occurred in the interim. Therefore, while NBER working papers can be valuable resources, they should be used judiciously, with an understanding of their provisional status in academic discourse.

thank you for your answer