  Recently we uploaded to the arxiv a paper entitled: Improving the Economic
Complexity Index. There, we compared three metrics of the knowledge intensity
of an economy, the original metric we published in 2009 (the Economic
Complexity Index or ECI), a variation of the metric proposed in 2012, and a
variation we called ECI+. It was brought to our attention that the definition
of ECI+ was equivalent to the variation of the metric proposed in 2012. We have
verified this claim, and found that while the equations are not exactly the
same, they are similar enough to be our own oversight. More importantly, we now
ask: how many variations of the original ECI work? In this paper we provide a
simple unifying framework to explore multiple variations of ECI, including both
the original 2009 ECI and the 2012 variation. We found that a large fraction of
variations have a similar predictive power, indicating that the chance of
finding a variation of ECI that works, after the seminal 2009 measure, are
surprisingly high. In fact, more than 28 percent of these variations have a
predictive power that is within 90 percent of the maximum for any variation.
These findings show that, once the idea of measuring economic complexity was
out, creating a variation with a similar predictive power (like the ones
proposed in 2012) was trivial (a 1 in 3 shot). More importantly, the result
show that using exports data to measure the knowledge intensity of an economy
is a robust phenomenon that works for multiple functional forms. Moreover, the
fact that multiple variations of the 2009 ECI perform close to the maximum,
tells us that no variation of ECI will have a performance that is substantially
better. This suggests that research efforts should focus on uncovering the
mechanisms that contribute to the diffusion and accumulation of productive
knowledge instead of on exploring small variations to existing measures.
