Neuman contends that quantitative methods are not about complicated statistics but simply about a concern for quantity. Quantitative methods are based upon the fundamental question -- how many of them are here? In itself that question is content free and value free, but the whole point is to apply it to data that are highly social and political in content. (Neuman, 1998, pp. 55-57) Harrison, Lisa. (2001) Political and Social Research: An Introduction. London: Routledge

Harrison (2001) discussed the concept of reliability in her work Political and Social Research: an Introduction. Harrison argued that a reliable indicator or measure gives you the result each time the same thing is measured (as long as what you are measuring is not changing). Reliability means that the information provided by indicators (e.g. The questionnaire) does not vary as a result of characteristics of the indicator, instrument or measurement device itself. A measurement has high reliability if...

Reliability means repeatability, consistency. Harrison contends that no measurement is perfectly reliable, so you will never get exactly the same result every single time, but the more similar the results are, the more reliable the measurement is.
Pennings, Paul, Hans Keman and Jan Kleinjihenhuis. (1999) Doing Research in Political Science: Introduction to Comparative Methods and Statistics. London: Sage Publications.

Validity is the degree of fit between a construct and indicators of it. It refers to how well the conceptual and operational mesh with each other. Validity is part of the dynamic process that grows by accumulating evidence over time and without it, all measurement becomes meaningless. There are different type of validity that includes face, criterion, content, external, internal and construct. These types were discussed by Harrison (2001) and Pennings et.al (1999).