As mentioned earlier, the union of code deltas and code coverage constitutes CBTM, and the goal of CBTM is to help you increase the effectiveness of your testing. Running a test takes a certain amount of time, and time is a precious commodity in the software industry, because there is a limited window of opportunity for a given product to take hold before it comes under fire from competitors.

Another thing to consider is the ever-changing environment of the computing industry. Assuming your product will work tomorrow simply because it worked today is dangerous because a new operating system or server application could become popular and your program might not work correctly when integrated. Figure 2.4 shows a simplified version of the different layers existing on a computer. A defect present in a lower layer bubbles up to the layers above. The problem is that the consumer does not always know to whom they should attribute the defect. Imagine the customer service call starting with, Your program was working yesterday, but when I installed a new operating system today, it ceased to work. The customer might not understand that the problem lies in faulty operating system code rather than faulty product code.

Figure 2.4: Simplified Computing Layers

CBTM helps maximize the amount of work your test suite is doing in order to take advantage of that limited time. Chapter 1 cited the reduction in the number of tests as the key to increasing effectiveness with CBTM...