Field Sources:

Include living persons who have the knowledge about or have been  in  intimate contact with  social conditions and changes over a considerable time. These People are  in  a position to describe not  only  the existing state of  affairs (e.g. patient tells about his conditions) but  also  the  observable trends (e.g. see how the 1CU nurse handles modem gadgets) and significant milestones in  the society. Field sources also include conditions, environment and events that are observable and measurable, such as sanitary conditions, humidity. Most  survey studies and all experimental studies use field sources for data collection.