How to Buy a Car Like a Systems Engineer – Part 4: Some Basic Decomposition



In this post we will develop the requirements a little further. Based on the gross cost estimates in the previous analysis we have, for all intents and purposes eliminated the airline architecture. When we perform the same kind of analysis for the two other possibilities- renting or buying a replacement vehicle- we see that the cost of air travel is significantly out of line.

Looking at the cost of renting a mid or full size vehicle we can use internet quotes to determine that the 3 day rental we would most often need (pick up on Friday morning and return on Monday morning) would cost about $140 per trip. The car pickup would necessitate a cab ride on each end ($15 each way or $30 per trip). At $170 per trip the yearly cost for 45 trips would be $8250. This is less than 1/3 of the yearly airline ticket cost.

This thinking gives us a clearer, more detailed picture of the behavior between the alternatives. Where the high level behavior of providing transportation is performed in the “new private car” allocation by the owner driving the car door to door, the behavior for the rental car involves booking a cab on both ends of the trip and taking it to and from the rental location. This is a good example of how work in one domain (testing the rental option against the cost requirement) can point to work in another (functional behavior). This movement is helpful and should be encouraged.

A rough estimate of the ownership cost (a mid or full size car purchased used at low mileage) shows it to be even lower in cost. The car payments would run about $400 per month. Maintenance- an item not needed for the rental option- would amount to about $1000 per year yielding at total of $5800. Again, this will point to a behavior refinement- this time in the behavior for the new car option. The owned automobile must be taken in for servicing and monitored for performance by the owner. Both activities will require time and resources. These considerations are surfaced by the thinking about the total cost of ownership.

As we decompose the requirements, behavior and physical architecture our analysis advances systematically and our confidence in the developing solution increases.

The Rest of the Series: Check back as we explore the rest of the process.

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