Perhaps the greatest challenge in social sciences research design is the difficulty of measuring the impact of the independent variable that you are trying to study. The social sciences look at the very messy human world, which has all sorts of intervening variables that can make it very difficult to determine whether a change in an independent variable has had an impact.
For example, let's say you are looking at the implications of the election of black leaders for black communities in general. You want to know if having a black mayor elected in a given city will end up actually helping the African American community in that city. This is very difficult for a number of reasons, but the most important one is the fact that the election of the black mayor does not happen in a vacuum. There are all sorts of other factors, such as the state of the national economy, the trends in the local economy, and the political constraints within which the black mayor works that can make it very difficult to determine how much of an impact the mayor had and how much other variables caused whatever changes you might see in the overall welfare of the African American community in that city.