Any job description containing 20-30 tasks is actually more like a part of an operational manual, which serves a different purpose. Job descriptions should refer to the operational manual, or to 'agreed procedures', rather than include the detail of the tasks in the job description. If you include task detail in a job description you will need to change it when the task detail changes, as it will often do. What would you rather change, 100 job descriptions or one operational manual?