There aren't too many ways to get one number, but you are restricted to whatever measurement interval is available in the data. I couldn't find that reference either (Riccardi, 1995) so I don't know how they did their averaging.
But suffice to say it's not thousands of reporting stations worldwide, weighted for area and averaged. Because of that fact, the extremes like 1998 won't show up. You are welcome to point to some error bars and repeat (but not explain) some statistics, and I will try to critique it.
Still waiting for some temperature proxy measurements showing average (about a half century or more) cooler conditions anywhere on earth during the MWP. So far the Vikings are winning.
I don't know how they did their averaging
An appeal to ignorance? (I have learned something from this discussion.)
Regarding the MWP. The best I can do right now is to refer you to
this document. I will note that it is not a peer reviewed paper however in the figure it shows 4 reconstructions of Northern hemisphere temperature (the solid lines). The reconstructions are from peer reviewed journals. Each reconstruction used a different set of proxy data (although there may have been some overlap), different methodologies and different authors. Go wild!