“Simpson’s paradox (or Simpson’s reversal, Yule–Simpson effect, amalgamation paradox, or reversal paradox), is a phenomenon in probability and statistics, in which a trend appears in several different groups of data but disappears or reverses when these groups are combined.
—s.v. Simpson’s Paradox, Wikipedia.
An example using arithmetic from the Stanford Encyclopedia of Philosophy:
1/5 < 2/8
6/8 < 4/5
7/13 > 6/13