Valid research reveals the true picture, data that is highly valid tends to be qualitative, it seeks to provide the researcher with a deep understanding of their research object.
Validity can be undermined by the use of closed questions which limits respondents answers and also the hawthorne effect which means when subjects of experimental study change their behaviour because they are being observed
Reliability
In research the reliability of a method refers to the extent to which, if the same study were repeated, it would produce the same results.
For this to be the case samples need to be representative, questions or processes need to be uniform and data would generally need to be quantitative
Generalisability
Whether the conclusion can be applied to only the specific group being studied or whether they can be generalisable to the wider public
Representativeness
The extent to which a sample mirrors a researchers wider target population and reflects its characteristics