Hidden Pairs grow into Hidden Groups.
If three digits in a house appear only inside the same three cells (a Hidden Triple), or four digits inside four cells (a Hidden Quad), those cells are reserved for exactly that set of digits.
Before this: Learn Hidden Pair first.
Why it works
Same ownership logic as a Hidden Pair, just with a bigger club.
N digits are owed to the house. They only have N shared seats. If any of those seats took an outsider digit, at least one club member would have nowhere to go.
So strip every candidate that is not in the club from those N cells.
Demonstration
Think of a Hidden Pair, but allow three or four digits that share the same three or four cells. The house still owes every digit.
Here 5, 6, and 8 only live inside these marked cells — shrink their homes until that matching set of seats remains. That is a Hidden Group.
Those cells must contain exactly 5, 6, and 8 (in some order). Extra marks are impossible — an outsider would leave a club member homeless.
Remove the extras (here 1, 2, 3, and 9) (red). You reserved the seats for the whole group.
Example 1
Find N digits that appear only in the same N cells of a house.
1, 5, and 9 form a Hidden Group here.
Clear non-group candidates from those cells (here 2, 3, 4, and 6) (red). The house owes those digits exactly those seats.
Example 2
Find N digits that appear only in the same N cells of a house.
4, 5, and 6 form a Hidden Group here.
Clear non-group candidates from those cells (here 2 and 9) (red). The house owes those digits exactly those seats.