We review the essential characteristics of the three different approaches to specifying
a sampling strategy; the design-based approach, the model-assisted approach and the
model-based approach. We then describe a unified framework for survey design and
estimation that incorporates all three approaches, allowing us to contrast them in terms of
their concepts of efficiency as well as their robustness to assumptions about the
characteristics of the finite population. Our conclusion is that although no one approach
delivers both efficiency and robustness, the model-based approach seems to achieve the
best compromise between these typically conflicting objectives.