How a bridge champ beats the market discusses how a 10 time national champion Bridge player uses his skills in that area to run a successful (if small) hedge fund. One paragraph stuck out when reading it.

The connection with bridge, says Moss, is the ability to decide what data matter, and then to have the judgment to act on it. “In bridge and investing, you are constantly being bombarded with an enormous amount of information,” he says. “The key is seeing all the possibilities.” There’s more. In bridge, an opponent’s tempo of play will tell an expert more than an amateur. Similarly, in investing, knowing which market indicators to monitor, and when, is more critical than watching every piece of information.

The last sentence is actually the most important one to testing. We can’t possibly test every permutation of inputs into the system or monitor every single response from it. We can however understand our [and our business’] context, what our mission is for testing, what things might also lead to new, interesting information.

And knowing how, when and why do that is what separates the professional testers from the amateurs.