Researchers want to investigate how digital wellness interventions impact the occupational stress of healthcare professionals. A researcher wants to study whether junior doctors report lower stress levels when using a specific daily 10-minute guided mindfulness mobile application compared to those who take a standard self-directed break. The study is to be conducted at a single, highly-pressured urban teaching hospital with 280 junior doctors currently on rotation.
Explain two factors that could affect the external validity of this study.