9.2 Limitations: External validity
Externally validity refers to the ability to generalise the results to other groups in the population apart from the sample studied (Sect. 3.6). For a study to be externally valid, it must first be internally valid.
External validity refers to the applicability or the generalisability of the results to the intended population (Example 5.10), which depends on how the sample was obtained: results from random samples are likely to generalise to the population and be externally valid. when appropriately analysed. (The analyses in this book assume all samples are simple random samples.) Furthermore, results from approximately representative samples may generalise to the population and be externally valid if those in the study are not obviously different than those not in the study.
Example 8.1 (External validity) A New Zealand study (Gammon et al. 2012) identified (for well-documented reasons) a particular group to study: ‘women of South Asian origin living in New Zealand’ (p. 21).
The women in the sample came from a study
…which investigated the health and lifestyle of women of South Asian origin living in New Zealand. Subjects […] were recruited using a convenience sample method throughout Auckland, which is New Zealand’s largest city, and the city in which most South Asian immigrants settle…
— (Gammon et al. (2012), p. 21)
The results may not generalise to the intended population of ‘women of South Asian origin living in New Zealand’ because all the women in the sample came from only one city in New Zealand (Auckland), and the sample was not a random sample.
The results will not generalise to all New Zealand women, but this is not a limitation: the target population was only ‘women of South Asian origin living in New Zealand.’ The researchers did not intend the results to apply to all New Zealnd women.