Count of Counts

Greg_Deckler  No, that's the point - any slicer or filter settings applied to the underlying table (tVisits) don't propagate through to the derived table (tSummary).

But I think I've found the answer myself - use multiple 'group by' clauses in the SUMMARIZE instructions, then apply the same slicers/filters to the derived table as to the source. So I can pass through the Year like this:

tSummary =
SUMMARIZE(
   SUMMARIZE(
      tVisits,
      tVisits[Person],
      tVisits[Year],
      "DistinctSites",
      DISTINCTCOUNT(tVisits[SiteVisited])
   ),
   [DistinctSites],
   [Year],
   "CountVisitors",
   DISTINCTCOUNT(tVisits[Person])
)

Not sure if this is the most elegant solution, but it works..