Because it is known to be a stressful industry, and because calmness is a key attribute needed by both pilots and cabin crews, professional stress management has long been a part of the aviation industry's approach. Few industries have put more into how they manage the stress levels of employees. As such, much can be learned about the way in which the sector has innovated with methods of assessing and managing stress. Essentially, aviation operators use several performance-based quantitative measures to assess the performance of employees, which offer insights into their stress levels. The sector also uses more qualitative tools, such as self-assessment questionnaires which employees will fill in about themselves several times a year. Another approach airlines and cargo flight operators use is to assess employees physiologically. In the case of pilots, this can mean monitoring their heart rate and taking their blood pressure, for instance.