Milestones, Averages and Ranges, Oh My!

"How many words should my child have?" Perhaps you've asked this question or perhaps you've bore witness to the countless Facebook group threads where parents compare how many words their child has with one another (can you sense my disdain for these?). You've probably seen a few versions of this chart, too. You may have even noticed variations in the numbers as well (this has to do with the source of the data and the writer's interpretation of that data).

This particular data is from Fenson and colleagues (1994) based on 1803 children whose parents completed the MacArthur-Bates Communicative Development Inventories (MB-CDIs). These authors reported data from children scoring at the 10th, 25th, 50th, 75th and 90th percentiles and discussed the trends in growth across these levels. I've presented this data below. You'll notice, however, that I've also included a column for variability because variability is inherent to the discussion of expressive vocabulary development in that there truly is such a wide range of normal (check out how dramatic the variability in number of words starts to become at 15 months!) This has everything to do with the fact that a child's trajectory of vocabulary development is so unique. Some children are gradual word learners, others have word explosions or 'spurts' early on and another subset have spurts later on. What we look for as Speech-Language Pathologists is whether or not your child is meeting specific language milestones. Milestone data is based on what 90% of children at that age are doing. So this is in essence, the minimum of what we look for. At 18 months, this starts to become particularly relevant because this is when we tend to identify children as being late-to-talk (I.e. they have fewer than the milestone of 20 words). This signals to us that these children may benefit from some support in their language skills.

The MacArthur-Bates CDI is one tool that is often used to get an idea of the size of a child's vocabulary. When using this tool, parents record from 680 word options, how many words their child is using. But just like anything, it is not a perfect tool and depends on a lot of factors including parent recall of all of the words their child is using (imagine how hard it is to keep track of hundreds of words!) as well as one's definition of a 'word'. Nonetheless, tools like this help us to gauge where your child is relative to their peers. Normative data such as the data from the chart above, provides us with that reference point to compare to. In addition to number of words, we also look for other characteristics of expressive language such as word combinations, which we expect to see at around 24 months (As an aside: children tend to begin to combine words when their vocabularies reach roughly 50 words). If a child is not yet combining words despite having an adequate vocabulary size, this may also signal to us that they could benefit from some support.

SLPs use the best available data to make decisions about whether or not a child would benefit from intervention. If your child is not reaching his or her language milestones, it is always best to get a second opinion by speaking with your child's primary care provider or speaking directly to an SLP. The numbers and data out there can be so confusing and it is my goal to educate parents on important terminology so that they can be better equipped to make decisions about their children's futures. Facebook threads are not the place to be comparing vocabulary sizes and in my experience, can make parents feel quite inadequate. We have also been trained to think that 'Average' is not good. In the chart above, I've included the average vocabulary sizes across age groups. This represents what 50% of children at that age are producing. Trust me when I say that average is GREAT! It's right where you want to be. (For a more in-depth discussion of percentiles, check out this Blog Post).

So please, steer clear of the social media chats and use empirical evidence as your reference point.

-- Meg

Reference:

Fenson, L., Dale, P. S., Reznick, J. S., Bates, E., Thal, D. J., Pethick, S. J., Tomasello, M., Mervis, C. B., & Stiles, J. (1994). Variability in Early Communicative Development. Monographs of the Society for Research in Child Development, 59(5), i–185. https://doi.org/10.2307/1166093