Discrete choice models generally require data to be structured in one of two basic formats: the case-only (“idco”) format or the case-alternative (“idca”) format. This are commonly referred to as IDCase (each record contains all the information for mode choice over alternatives for a single trip) or IDCase-IDAlt (each record contains all the information for a single alternative available to each decision maker so there is one record for each alternative for each choice).
In the idco case-only format, each record provides all the relevant information about an individual choice, including the variables related to the decision maker or the choice itself, as well as alternative related variables for all available alternatives and a variable indicating which alternative was chosen.
Example of data in idco format¶ caseid Income Alt 1 Time Alt 1 Cost Alt 2 Time Alt 2 Cost Alt 3 Time Alt 3 Cost Chosen Alt 1 30,000 30 150 40 100 20 200 1 2 30,000 25 125 35 100 0 0 2 3 40,000 40 125 50 75 30 175 3 4 50,000 15 225 20 150 10 250 3
In the idca case-alternative format, each record can include information on the variables related to the decision maker or the choice itself, the attributes of that particular alternative, and a choice variable that indicates whether the alternative was or was not chosen.
Example of data in idca format¶ caseid Alt Number Number Of Alts Income Time Cost Chosen 1 1 3 30,000 30 150 1 1 2 3 30,000 40 100 0 1 3 3 30,000 20 200 0 2 1 2 30,000 25 125 0 2 2 2 30,000 35 100 1 3 1 3 40,000 40 125 0 3 2 3 40,000 50 75 0 3 3 3 40,000 30 175 1 4 1 3 50,000 15 225 0 4 2 3 50,000 20 150 0 4 3 3 50,000 10 250 1
In the abstract, these two data formats are completely interchangable: any data that you can express in idco format, you can also express in idca format, and vice versa. The preference for one format or another is generally for convenience only.