Well I am not sure this thread went the way I had hoped. But to be clear -- the ability to do math in one's head while a good tool to have (even if just to get a date) is exactly the opposite of what I was saying.

And whether what I was saying was conventional wisdom -- I will reiterate that using an explicit model is the best way to avoid the screw-ups when trying to do it all in your head. I will leave with a passage from the referenced intelligence book:

Another significant question concerns the extent to which analysts possess an accurate understanding of their own mental processes. How good is their insight into how they actually weight evidence in making judgments? For each situation to be analyzed, they have an implicit “mental model” consisting of beliefs and assumptions as to which variables are most important and how they are related to each other. If analysts have good insight into their own mental model, they should be able to identify and describe the variables they have considered most important in making judgments.

There is strong experimental evidence, however, that such self-insight is usually faulty. The expert perceives his or her own judgmental process, including the number of different kinds of information taken into account, as being considerably more complex than is in fact the case. Experts overestimate the importance of factors that have only a minor impact on their judgment and underestimate the extent to which their decisions are based on a few major variables. In short, people’s mental models are simpler than they think, and the analyst is typically unaware not only of which variables should have the greatest influence, but also which variables actually are having the greatest influence.

Thanks

Matthew