They also measured:
the children’s ability to process speech – they showed the children pairs of pictures (e.g. dog/baby) and then said a sentence which named one of the pictures. They watched to see where and how quickly the children looked when they heard the word that named one of the pictures. This allowed them to determine how efficiently the children processed the speech they heard.
the children’s vocabulary – they determined how many words the children knew at age 24 months.