For a variety of historical reasons, a common way to put data in models is through the use of Lookup tables which use Time as the input. While it is possible to do this in Vensim, it is not a recommended practice. The entry of data into Lookups can easily lead to the introduction of errors. Because it is so easy to create exogenous data using any text editor, you should do this and import it as a dataset instead of using Lookup tables as drivers. If you are using data to drive a model, the different interpolation modes offered for Data variables can prove very helpful. If you are comparing model behavior with data, Vensim has built in logic to handle missing data points. The Strip Graph and Graph tools also work best when a data series has the same name as a model variable, something that is not possible when the data is entered through a Lookup.
