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Introduction

Ideally, when doing social scientific research we'd study the full population of interest. However, given the limited resources available to political scientists for research (for example, the annual budget for the political science research provided by the U.S. National Science Foundation is around $10 million [1]), as well as practical considerations, realistically large-scale research must make do with a subset of the population or a sample. Careful sample design, however, is necessary to ensure that our sample will be representative of the population of interest.

Full population studies: the census

With relatively small populations, or for specialized purposes, it may be reasonable to study the full population. Such a study is referred to as a census. The most famous examples of censuses are the regular censuses conducted by most countries to catalog the characteristics of their populations, such as the United States Census conducted every ten years and the Canadian census conducted every five years. In democratic countries, these censuses are important because they determine the basis of representation in national and subnational legislative bodies; they can also be important for the allocation of societal resources and for understanding the nature of social and economic problems faced by the population at large.

However, social scientific researchers almost never have the resources sufficient to conduct such a census; for example, the 2010 U.S. Census cost an estimated $13 billion U.S., or $42 U.S. per respondent, and required the hiring of thousands of enumerators and data processing employees. A census may be practical in more limited settings, such as a small community or an institutional setting (such as a university or college) with a relatively small population. A census may also be an appropriate approach to comparing the characteristics of large political units (for example, studying countries or subnational political units such as states and provinces) or the members of political bodies (such as a parliament or other legislature).

Sources of error in sampling

Selection bias...

Non-response bias...

Glossary

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