Statistical learning is an important element of language acquisition. A basic unresolved question is, what are the units over which statistics are calculated? In a corpus study and two infant behavioral experiments, we show that varying the units that are used greatly affects learning. Using words as units, nouns are easier to segment from continuous speech than verbs. However, if a highly frequent morphological element such as -ing is also treated as a unit, noun-verb differences disappear, in both corpus analysis and behavioral studies. These results suggest that infants can compute statistics over units other than words and syllables, and theories of statistical learning need better accounts for why some units are tracked and not others.