Validity refers to the extent to which the data measures what it claims to measure. Internal validity refers to whether there is a causal relationship between the phenomenon being studied and the factors that we think causes it. For example, a study may seek to establish a causal relationship between women’s fertility and women’s level of education. The study is internally valid if the researcher can control for the effects of other possible factors, such as access to contraception. External validity refers to whether the results of a study can be generalized to other settings (ecological validity), other people (population validity) and over time (historical validity). For example, if the fertility-education study is conducted in a particular country in a particular year, an externally valid study may yield similar results for a different country, with a different group of respondents, a decade later.