This is generally false unless the group has a clue what's going on. A group of non-medical people will do poorly in diagnosing a disease, for example. Likewise when about 50% of the people in the US do not know that the Earth goes around the Sun in once per year, such a crowd is poor at doing most scientific stuff.
In market analysis, the crown is probably fairly knowledgeable relative to the expert anyway. This is one reason for peer-review and for having committees. It's true that a single expert can miss things that a group (by virture of its varying experience) will detect.
This is generally false unless the group has a clue what's going on.
You got me. I misrepresented the premise. I should have said "usually," and I did not explain, as the author did, that the groups had to have some understanding of the problem or question at hand. He did not claim that any group will always make better decisions than any single expert. I think I posted before I'd gotten my second cup. It's pretty obvious that market analysis would find polling data useful, but many of the examples cited had to do with predictions regarding concrete, verifiable facts: locating a sunken submarine, or guessing the dressed weight of a steer in a county fair contest. By averaging the best guesses of a number of experts, a point was chosen on a map that proved to be within 450 yards of where the sub was finally located. In the second example, the average of the attendees' guesses was more accurate than the best estimate of any individual expert.
In other words, I guess, more heads are better than one, given a minimal degree of expertise.
He also points out that professional groups often inhibit decisions and undermine innovation. What he termed "group think" or conformity to professional standards is behind this. I suspect that this is true, that education is only partly about knowledge; it also involves indoctrination of conventions and values. I agree with shared values and conventions if they're good and sensible. But it strikes me that a particular kind of values has taken hold.