Non-Participant Observation is where researchers take a ‘fly on the wall approach’ and observes individuals and groups without getting involved in the life of the group. You would have come across this type of method in the form of the OFSTED lesson observation.
Non-Participant Observation can either be structured or unstructured – the former is where you have an ‘observation schedule’ and look for certain things happening, the latter is where you just observe and note down anything that stands out.
NPO can also be overt (like the OFSTED inspection) or covert, in which case it would either involve some infiltrating a classroom, or a workplace and observing without people being informed (as you can imagine this would be quite difficult to do in practice, or more realistically it might involve the use of hidden cameras to film covertly.
Some General Advantages of Quantitative Non Participant Observation
They have good reliability and are good for making comparisons
They are relatively quick and cheap to carry out
Some General Disadvantages of Quantitative Non Participant Observation
They lack validity because you are less able to ask why people are acting in the way that they do compared to participant observation
Ethically they can be dis empowering for respondents (OFSTED inspections)