We rate a child's speech on 'intelligibility', this refers to how well a stranger can understand what they're saying in words and conversation.
By 2 years of age, a stranger (e.g. shop assistant) should be able to understand 50% of the words and sounds your child has said.
By the age of 4, an unfamiliar person should be able to understand everything your child says.
ARTICULATION
Articulation refers to how we make sounds with our tongue, teeth, lips and voice. Children develop their ability to say new sounds between 2 and 6 years of age. An articulation delay is where the child may not be able to produce a sound by the expected age or they produce the sound in a different position in their mouth.
PHONOLOGY
As children grow, the way they say sounds change. Phonology is a pattern of sound errors which change as children develop.
A phonological delay is where the pattern of error continues beyond the age by which they are expected to be said correctly.
McLeod, S., & Crowe, K. (2018). Children’s Consonant Acquisition in 27 Languages: A Cross-Linguistic Review, American Journal of Speech-Language Pathology, 27, 1546-1571.
Phonological Processes Infographic Generated by Stepping Stones Speech Pathology
Bowen, Caroline (2011) Elimination of Phonological Processes in Typical Development.
Linguisystems (2008) Phonological Pattern Suppression by Age. https://www.linguisystems.com