Macroeconomics, Why is this subject so inexact and what can be done to improve this?

Interesting questions. But you wouldn't think to ask these questions of English professors or even Historians. Why do you expect Economists to do any better? It is certainly the case that economists claim to be able to do better, but that doesn't mean they can be trusted.

Economists like to claim that theirs is a real science, but we all know better. It does provide a useful collections of techniques and models to think about certain elements of human behavior, but human behavior is so complex that no one can predict it in general.

Making things much harder is that most economists have a personal stake in the results. That means they look for the evidence that supports their personal world view and tend to ignore or deprecate the evidence that doesn't. For example, in a Congressional hearing on the financial crisis, Alan Greenspan, once head of the Federal Reserve, said,

"that he had put too much faith in the self-correcting power of free markets and had failed to anticipate the self-destructive power of wanton mortgage lending.

“Those of us who have looked to the self-interest of lending institutions to protect shareholders’ equity, myself included, are in a state of shocked disbelief,”

Yet how many times had he seen other corporations operate in self-destructive ways? How many other bubbles has he seen? and how many that happened before his time had he studied? How could he have been surprised unless he deliberately ignored all the evidence that had passed before him. (And it is interesting that other economists in the Bush administration still claim to believe they have no responsibility for the mess we are in, that they behaved exactly as they should have.)

Economics grew out of a desire to influence political decisions. This means that it is, of necessity, bound up in people's political beliefs and values.

For example, all the evidence says that giving more money to the poor stimulates the economy more effectively than giving it to the rich. Thus food stamp programs, medical care for the poor, etc. are more effective than tax cuts for the rich. Even the Republican economists (Martin Feldstein was Reagan's economics advisor) agree:

http://www.epi.org/economic_snapshots/entry/webfea...

http://online.wsj.com/article/SB121798022246515105...

http://www.cbpp.org/1-14-08bud.htm

But for many conservative economists, "welfare" is so politically unpalatable that they refuse to countenance it. So they continue to propose tax cuts:

http://www.cnbc.com/id/28673775

http://www.nytimes.com/2009/01/26/us/politics/26ta...

So what can we do about this? It is not at all clear.

Obviously, we can't conduct real experiments at the real scale. It is not clear whether we can successfully divorce normative economics from positive economics, though we should try:

http://en.wikipedia.org/wiki/Normative_economics

I also think we should be teaching economics to more people earlier in their education, and when we do teach economics, make it clear that it isn't like physics with simple right answers.