The final and very important principle to understand about economics is that the measurements commonly used by economists are incomplete. This creates distortion in how economists assess the relative health and performance of various economies.
For example, one of the main measures – Gross Domestic Product (GDP) – is the sum of all revenue. It is an attempt to measure the volume of trade, and as such, output. However it is vulnerable to distortion. Imagine if every person paid the neighbour to the left of them $1 to say hello to them every morning. This would be a circular trade where the net cost and revenue would be zero. However the GDP increase would be $356 per person. You could rightly argue that this would never happen, but it is also true that there are plenty of other interactions that are not measured. Childcare is perhaps another way to illustrate this point. If you use a relative and have an unofficial transaction there is no impact on GDP. If you use a company, then GDP goes up. The same goes for cleaning, cooking, etc. Also the source of most GDP data is governments, and they rely heavily on data submitted by individuals and companies as part of their tax collection process. In some countries this represents only a fraction of the population. For example, in Pakistan less than a quarter of the population submit a tax return.
The fact that economic measures being used do not capture everything they are attempting to measure is just one of the issues. Another issue is that the economies are defined by geopolitical boundaries rather than actual economic ecosystems. As economics is essentially about reward for effort, and the values of both of these things vary enormously between individuals and cultures, it is unlikely that there will ever be a truly valid measure of economic activity.
This is not to discourage economic measurement. There is still much to be gained from the economic data that continues to be collected. The value of understanding this principle is in the interpretation of the measurements. For example, an economy that is in transition from a relatively unmeasured state to a measured state, is going to have a higher growth in GDP than one in which a high level of measurement is already established.