[wayward]

The 24% failure rate cited by Planned Parenthood includes ALL couples who claimed to be using fertility awareness, from those who had taken classes and were using a modern method, to those who were just guessing. IIRC, over 80% of the couples were not properly trained in a modern method of fertility awareness. Not surprisingly, they had very high pregnancy rates. Someone did the math (can't find the article) and figured untrained users had about a 28% pregnancy rate while trained users had a 7% pregnancy rate. (80 * .28 + 20 * .07) = 23.8 The 7% pregnancy rate is comparable to the real-world use of the Pill.

As for your system analogy, if your automated system was a significant resource hog or ran the risk of corruption of data or crashing the system, would you use it? You focus on efficacy without factoring in side effects.