The Heisenberg uncertainty principle is also frequently, confused with the "observer effect". The uncertainty principle actually describes how precisely we may measure the position and momentum of a particle at the same time – if we increase the precision in measuring one quantity, we are forced to lose precision in measuring the other. Thus, the uncertainty principle deals with "measurement", and not "observation". The idea that the Uncertainty Principle is caused by disturbance (and hence by observation) is not considered to be valid by some, although it was discussed in the early years of quantum mechanics, and is often repeated in popular treatments.