SI : International System of Units (meters, kilograms, etc.). Most countries use SI units, except USA and a few more.
What are the UX reasons behind countries not switching to SI units even though they are a clear improvement?
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This is largely a case of path dependency where once a standard has been set, it is incredibly difficult to change. In fact portions of the metric system had been in use in mathematics for about 700 years before it became adopted by France after the French Revolution. After that Napoleon brought the Metric system to all the countries that the French invaded in the Napoleonic wars, and once they had changed to the metric system, there was little reason for them to go back. It then became a bit of a trend as countries who mainly interacted with other metric countries changed over.
The few vestiges were mostly in the English speaking world which took much longer (usually some struggle for independence was a trigger) to switch over, with the USA and UK being the last real bastions of stubbornness.
There was a huge push many years ago to switch the US over to the metric system, but US manufacturing opposed it as it would mean high costs in retooling. The one change is that the inch was modified so that it was easier to convert accurately between inches and metres.
It will take a lot of political will for the US and UK to switch over to the metric system. It's a better system, but to most people doesn't affect their lives enough to care about it.
In short, the general consensus in the USA and UK has to be that the benefits of switching to the metric system outweigh the difficulties that it will bring, and until that happens (or there is actually some political leadership - yes, not very likely) it is unlikely to change.
The UX lesson in this is that you have to take into account switching costs when making a change to a system. Having a better system isn't enough. It needs to be perceived as better to those making the decisions - whether it's an individual or a company.
It would be a tremendous short term expense to completely change over. Existing records would need to be changed, as well as instruments, software, etc. to use the new system. Add to that the human costs of retraining and the errors incurred by incomplete retraining and confusion.
Even if in the long run the change over costs are more than recouped, the short term expense makes it politically difficult in a country as large as the U.S.