Unlike in any pure science where certain universal, everlasting rules influence what is to be expected of a situation (like say, under any circumstance 2 + 2 will always be 4) in social science nothing can be taken for absolute certain. This is because social science deals with humanity, which is inherently transient. Taking this element of transience into consideration social scientists have to use not just quantitative but also qualitative methods of study.
A Qualitiative method of study differs from the usual quantitative methods (used in the study of pure sciences) in the sense that it tends to be far more subjective. This is in sharp contrast to the methods used in pure sciences which are exclusively objective. The research methods for social sciences are a combination of both quantitative and qualitative methods. An example of such a method is the Intelligence Quotient (or IQ) test which helps us quantify a certain non-specific capacity or quality of an individual.
Being open to qualitative methods of study also allows social scientists to incorporate non-quantitative data (i.e. non-numerical data, such as words, images, gestures, impressions etc.) in their study. Since human beings tend to operate in terms of such data and not in terms of the terminology used in ordinary sciences this broader outlook is far more applicable in the study of social sciences. (Roberts, 2003)
In keeping with the methodology employed in social science researches the theories too are formulated and used keeping in mind that they arent necessarily applicable to all conditions. Be it psychology, history, political science or economics human events are most likely to influence the rules which define the basic theories of a social science subject. Success in the field of social science researches can therefore be achieved if and only if it is understood that the subject has no space or opportunity for water-tight methodologies or theories.
Roberts, O M; (2003); Creating Future: An Approach in Tomorrows Perspective; National Book Trust. 244