Sample Size

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Sample size and representativeness are two related, but different issues. The sheer size of a sample does not guarantee its ability to accurately represent a target population. Large unrepresentative samples can perform as badly as small unrepresentative samples.

A survey sample’s ability to represent a population has to do with the sampling frame; that is the list from which the sample is selected. When some parts of the target population are not included in the sampled population, we are faced with selection bias, which prevent us from claiming the sample is representative of the target population. Selection bias can occur in different ways:

Convenience samples: These include respondents who are easier to select or who are most likely to respond. This sample will not be representative of harder-to-select individuals. Samples from online panels are a good example of convenience samples. These panels are composed by individuals who have expressed interest in participating in surveys, leaving out individuals who may be part of the target population, but are not available for interviewing through the panel.

Undercoverage: This happens when we fail to include all the target population in the sampling frame. Many online panels work hard at avoiding undercoverage bias, but the fact remains that certain demographics are underrepresented. For example, it is difficult to field online studies targeted at the total Hispanic population in the US without using a hybrid data collection approach that allows us to reach unacculturated Hispanics, who are usually underrepresented in most online panels. Coverage bias is also found in phone surveys that use telephone list sampling frames that exclude households without landline access. As more households substitute cell phones for their landlines, obtaining representative samples of certain demographic groups is almost impossible without including cell phone lists in the sampling frame.

Nonresponse: Selection bias...