Apologies for the delay in replying.
Thanks to David & Austin for suggestions.
Before digging in and debugging in depth as David helpfully suggested
I thought I'd check Austin's suggestion that one of the files has zero
observations, and it appears that this is indeed the root cause, due
to an earlier merge and removal of anyone who didn't match in both
datasets, which in this instance was everyone).
Correcting the earlier merge meant that there were observations in the
file that is being used in the call to -joinby-.
Apologies for the (obvious!) oversight and thanks for the pointers.
Cheers,
Neil
--
"Only two things are infinite, the universe and human stupidity, and
I'm not sure about the former." - Albert Einstein
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