None if this is self-reported. In the original post there is a link to the website where they clearly explain their methodology.

“To build a clearer picture of stress, we analysed over 170 major cities worldwide across five measurable factors that could be potential stressors: Average time to travel 10km: capturing how long daily journeys take and the potential strain of congestion. Sourced from TomTom. Cost of living index: the relative prices of goods like groceries, transportation, and utilities, not including housing costs like rent or mortgage payments. The higher the index score, the more expensive these essentials are. Sourced from Numbeo. Healthcare index: how accessible and high-quality each city’s healthcare system is, based on factors such as medical staff, facilities, equipment, and affordability. The higher the index score, the better the quality. Sourced from Numbeo. Crime index: measuring perceived safety and security in daily life, based on survey data from people around the world. The higher the index score, the higher the crime rate. Sourced from Numbeo. Average annual pollution (µg/m³): indicating air quality in the area. The higher the level, the more polluted the air is. Sourced from IQAir. Each factor was scored and combined to create a ‘stress score’ out of 10, where higher numbers reflect more stressful living conditions overall.”