[ivalm]

Except in astro we also make predictions and theories are accepted when predictions turn out to be right (famously with GR, for example). The problem with macroeconomics is that economists are not capable of meaningfully predicting future macroeconomic indicators (GDP, inflation, whatever). This is why it's not an actual science.

[barry-cotter]

If you can make successful macroeconomic predictions you can get very rich so there are many, many people working on doing so. But you don’t just need to be right, you need to be right first because people with a good model acquire more and more resources from their own growth and from acquiring more outside investment until they reach the limits of the strategy they are using.

Astronomy is not the study of things that resist being predicted and self-modify to be unpredictable upon being successfully predicted.

All that said here are some things we know in macroeconomics; In the long run inflation is caused by an increase in the money supply; If you unexpectedly increase inflation you will have an economic boom as people are fooled by the short run disequilibrium between their expectations of the real value of money and what their nominal money now buys, likewise an unexpected fall in inflation will lead to a recession; You can have at most two of control of inflation, control of the interest rate and a floating exchange rate. If interest rates go up you will attract investment and your currency will appreciate unless you have capital controls. If you want to maintain the value of your currency you have to keep your domestic real interest rate very close to the global real interest rate, unless you have capital controls. If you choose to forego capital controls and have a floating currency you can control interest rates and inflation but you may have large inflows and outflows of capital; If you contract the money supply like the Federal Reserve did after the 1929 stock market crash (30% decline in monetary base, 30%!) you will have a recession.

[ivalm]

I agree that it might be fundamentally impossible to make meaningful macroeconomic predictions (because it is self correcting). But this is one of the arguments why macroeconomics might fundamentally not be a science (kind of like the art of writing fiction isn't a real science, although there are many commonalities/themes/etc one can study).

Re your third paragraph. You make many claims which may or may not be correct, but at the end of the day nobody has any kind of track record of prediction long term inflation rates of GDP numbers. This means that there is little value in counterfactual claims (if we did x then y would happen). Every study (such as your allusion to '29) are explanative, not predictive. Explanative studies have relatively little to do with what makes something a science (prediction).

Finally, this is not to say that there is absolutely nothing meaningful in economics. Surely if you arbitrarily set tax rate to 100% or 0% you would have negative consequences. Similarly, if Fed changes current rates dramatically, this would have consequences. It's just for "normal" changes in policy we have basically no predictive power other than something like nearest-neighbor search in historical observation.

[zwaps]

The accuracy of GDP prediction, which - remember - amounts to the task of predicting everything there is without having "completed the science", is pretty decent all things considered.

Look for example at GDPnow forecast vs. reality. GDPnow is very spiky, since it changes constantly, but its performance is overall pretty accurate, and during each period its data use more than halves the RMSE. In other words, it has predictive power.

[ivalm]

If you look at IMF predictions of world GDP [1] you can see that simply taking last year's value would have had lower error for most years and in average.

[1] -- figure 2 https://www.ft.com/content/60581224-3335-11e8-b5bf-23cb17fd1...