A measurement is a direct reading of data from the data's object itself, using a scaled measuring device. When done properly, it is the highest possible source for data and trumps all other sources. Citations for measurements should give a clear impression of how the measurement was taken and how it avoided possibilities for error.

Measurements taken visually, such as counting the floors or elevators in a building, are classified as personal observations and do not fall under this source type.

The scaled device is applied to the object itself. Taking dimensions from a map, drawing, or model is mediated by that representation and does not meet this direct-measurement definition. Counting visible floors or elevators remains personal observation.