Joseph Berkson (1899 – 1982) was initially traine as a physicist. Later in his career he became primarily concerned with studying statistics. In 1950, while working at the Division of Biometry and Medical Statistics, Mayo Clinic, Rochester, Minnesota, (1899 – 1982), Berkson wrote a key paper entitled Are there two regressions?. In this paper Berkson proposed an error model for regression analysis that contradicted the classical error model until that point assumed to generally apply and this has since been termed the Berkson error model. Whereas the classical error model is statistically independent of the true variable, Berkson's model is statistically independent of the observed variable. Carroll et al. (1995) refer to the two types of error models as follows: