@realjohnboy,
rjb, That's a good way to explain how we measure economic output in a nutshell; they are only best estimates by looking at total payroll for the employment stats, and consumer and commercial spending for all the products and services produced plus savings to measure GDP. They refine their numbers through the measurement of the circulation of money, and try to gauge the inflation rate by counting a "basket" of goods. Many people were confused about the inflation rate, because the cost of energy increased dramatically, but was not reflected in the inflation rate.
But the tools to measure both employment/unemployment, GDP, and inflation have been improved dramatically since 50-years ago, because of our ability to use computers and statistical analysis.
We still do not have good tools to measure future economic activity for various reasons including the effect of the world economy on our own, and the effects of high deficits on future economic activity.
During WWII, our deficits exceeded our GDP, but we were able to surmount that handicap to build the richest country in the world.
Future economic activity depends on the education of our society and other factors such as the cost of energy, supply of natural resources, and our environment. It also depends on how long foreign countries continue to buy our bonds, and the strength of our economy vs the world economy.
If there are tools to measure future economic activity, the purchase of stock would be more straight forward. Anyone with such a tool can become the richest person on this planet.
Finally, economics is not science; it's art.