the direct effect of a resource—such as selecting teachers for their subject and cognitive ability—is a more manageable policy instrument that is more likely to affect student achievement than, say, difficult-to-control indirect mechanisms such as teaching practices.

The examples in this chapter demonstrate a third characteristic of scientific knowledge generation and accumulation: the interdependent and cyclic nature of empirical findings, methodological developments, and theory building. Theory and method build on one another both as a contributor to and a consequence of empirical observations and assertions about knowledge. New knowledge gained from increased precision in measurement (say) increases the accuracy of theory. An increasingly accurate theory suggests the possibility of new measurement techniques. Application of these new measurement techniques, in turn, produces new empirical evidence, and so the cycle continues. This cycle is characteristic of the natural sciences, as illustrated in our example of differential gene activation, and also evident in social science in the measurement of economic and social indicators (deNeufville, 1975; Sheldon, 1975) and education measurement (National Research Council, 2001b).

A fourth and final characteristic that emerges from these examples is a comparative one: studying humans is inherently complex. Humans are complex beings, and modeling their behavior, belief systems, actions, character traits, location in culture, and volition is intrinsically complicated. Research challenges arise in large part because social scientists lack the high degree of control over their subjects that is typical in the “hard” sciences—for example, gaggles of molecules are better behaved than a classroom of third-graders. This observation is not intended to suggest that science is incompatible with the study of the human world. Nor do we mean to say that scientific work is fundamentally different in these domains (indeed, the main message of Chapter 3 is that the core principles of science apply across all fields). Rather, scientific inquiry involving humans is qualitatively more complex than inquiry in the natural sciences, and thus scientific understanding often requires an integration of knowledge across a rich array of paradigms, schools of thought, and approaches (Smelser, 2001).



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