Sunday, May 17, 2009

Economic Model Update

Though I have not posted for a while, a lot of work has been done on the model.

The first step was to build the simulation engine, which has been architected and implemented by Steve Noble. This engine is the platform within which we create populations of the objects: households (people), banks and homes, as well as the markets, government and external factors.

The next step was to create the behaviors for the various objects. We currently have a mortgage market up and running, in which the households request, and banks offer, mortgages. On a given simulated day, each bank evaluates all current household requests for a mortgage, based on the bank’s lending policy, and each requesting household evaluates any mortgages for which it is qualified. Each household and bank has its own unique way to evaluate a mortgage. I’ll describe these in a future positing.

We don't have houses in the model yet—we simply have the households request mortgages and, if the request is filled (the mortgage is preapproved), then the household has a probability of actually buying a house at the preapproved amount of the mortgage. The buying household then pays the down payment to the seller, and pays any points on the loan to the bank. The buyer then makes mortgage payments until the loan is paid off. The current loans are simple fixed interest rate loans.

While the addition of houses will allow for a supply-and-demand variation in housing prices, the current simple model already provides interesting behavior with respect to the business models of the banks. Banks make money on loan interest paid, but to make loans a bank needs deposits and has to pay interest on the deposits. These interest payment seat into the loan profits. The banks are competing for the households’ loan business, so they can’t charge too high an amount for loan interest. But if the deal on the mortgage is too “good” from the perspective of the borrower then the bank will get lots of business but lose money.

This example highlights that even when there is no risk of default in the loans, the banks still must balance offering attractive loans with making enough money to pay interest and turn a profit. If there is just one bank it is easy, but when there are multiple banks competing for the borrowers’ business it is not clear ahead of time which bank will win.

We have some more work to do to finish this first simulation, and plan to put it online within the next two to three weeks. Watch this space for an announcement when we do!

Meanwhile, here's a screenshot of the work in progress...