'Valid,' talking about the GDP measure, would be the question whether the GDP estimate is correct. Does it capture the real economy 100%? Now we know that a GDP measure of the U.S. economy, the Germany economy, the Norwegian economy, will never be correct. It will always be a little bit off. Some data–there will be some cheating, there will be some data which are questionable. But we know we are more or less within bounds, off a couple of percentage here and there. And so that would be the question of validity. As we’ve seen from recent events in Ghana, and also forthcoming events in Nigeria, the validity question is really huge in sub-Saharan Africa. We are talking about plus-minus 50 to 100% on GDP levels. This would maybe not be a problem if you were interested in change, as we were talking about: what one type of change has a causal effect on another, such as GDP, liberalization, and parity. The problem is if you have that the validity of the measure changes through time. So that would be if you equated this with your bathroom scale at home–it wouldn’t be such a big problem if your personal scale was off a pound or two, if you were basically just interested in measuring yourself on a weekly basis to see if you are gaining or losing. The problem that comes in is that of reliability, and that is if someone changes your scale in the middle of the night. And therefore you have a scale that shows an error in a different direction. And there you will have different problems talking about time series or changes over time. Another problem is that validity still remains with us even if the data was reliable, in that if you started comparing your own weight with that of the neighbor, who uses a different scale, then it would still be very different to determine who is the heaviest or lightest.
This is from Simon University Professor Morten Jerven in a podcast discussion with Russ Roberts of the Café Hayek on the unreliability of GDP as a measure to determine a nation's accessibility to multilateral aid or “concessional lending through the International Development Association (IDA), the concessional arm of the World Bank”, applied mostly to African countries.
Two points here: statistical GDP can hardly be relied on to reflect on the real or genuine economic conditions since it is subject to significant statistical errors, and importantly, vulnerable to manipulation. If an analysis is based on an inaccurate tool, then the result would most likely be misdiagnosis. So I'd be leery about mainstream's unwavering or pious devotion to statistical growth.
Second, liberalization is the best alternative to economic development