A while ago, I introduced Galtung’s (1967) AJUS system to classify distributions according to shape. The AJUS system can be used to reduce complexity.
Galtung also introduced the ISD system to describe changes over time in a similar manner. It can be applied to any situation where we have three points in time to characterize two periods during which changes may have happened. As with the AJUS system, the intuition is to ignore small and unimportant differences to focus on the bigger changes. While Galtung developed the ISD system for eye-balling, it remains relevant in the age of computers; the system just becomes more systematic.
The ISD system is available in my R package agrmt. Taking a vector representing the values at the three points in time, the isd() function gives the type and a description of the type as follows:
- Type 1: increase in both periods
- Type 2: increase in first period, flat in second period
- Type 3: increase in first period, decrease in second period
- Type 4: flat in first period, increase in second period
- Type 5: flat in both periods
- Type 6: flat in first period, decrease in second period
- Type 7: decrease in first period, increase in second period
- Type 8: decrease in first period, flat in second period
- Type 9: decrease in both periods
Reference: Galtung, J. 1969. Theory and Methods of Social Research. Oslo: Universitetsforlaget.
Didier Ruedin 