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.