This is for sure the most simple and, in a way, primitive model. Within one level data warehouses, we actually have just one operational database, where data is written and read, mixing those two kinds of activities. A virtual data warehouse layer is then offered to perform inquiry activities. This is a primitive model for the simple reason that it is not able to warrant the appropriate level of segregation between live data, which is the one currently produced from the process, and historical data. This model could therefore produce inaccurate data and even a data loss episode. 

This model would be particularly dangerous for data mining activity, since it would not ensure a clear segregation between the development environment and the production one.