Speech, like any other area of development, requires practice; therefore, errors will happen. Because of this, professionals typically start to assess articulation or speech sound acquisition after the age of three. Intelligibility (i.e., clarity of message) is often used to determine whether or not a child would be a good candidate for therapy. Flipsen's (2006) conversational intelligibility guideline is converted by dividing the child's age in years by four and converting the number into a percentage.
1 year—1/4 = 25% intelligible
2 years—2/4 = 50% intelligible
3 years—3/4 = 75% intelligible
4 years—4/4 = 100% intelligible