Two or more variables are said to be cointegrated if they are individually integrated
(or have a random walk component), but there exists a linear combination of them which
is stationary. The concept of cointegration was first introduced by Granger (1986) and
more formally developed in Engle and Granger (1987). Rigorous statistical treatments
followed in the papers by Johansen (1988, 1991) and Phillips (1991). Many further
developments and extensions have taken place with reviews provided in Johansen (1995),
Juselius (2006) and Garret, Lee, Pesaran and Shin (2006). The related unit root literature
is reviewed by Stock (1994) and Phillips and Xiao (1998)