Data taken from personal observation is anything with an unmediated relationship between the fact itself and the person who records it.

This is only a good source for certain kinds of data. For example, the year a building is built can be taken from personal observation if someone witnesses its completion (i.e. can pin the completion down to a certain year or month). Data which can be counted, such as the number of floors or elevators, can also be gathered this way, but data which requires measurement cannot.

When personal observation is valid for a data field, it is generally a very good source of the data.