How we know this

What the ages mean

Researchers find out when children start doing something by watching lots of children, or asking their parents, as they grow. Then they look at the ages when the children started.

On each page, the coloured bar shows the ages when most children in those studies started. The dot shows a typical age. Each study can measure this a little differently, so “The studies” on every page says exactly what each study counted.

We show the whole spread of ages, not just one average, because children vary a lot.

How sure are we?

Every page says how sure we can be, in one of four ways:

• Very sure. Lots of big studies in different countries agree.
• Fairly sure. Good studies agree, but mostly from a few places.
• Not sure yet. Only a few small studies so far.
• Researchers disagree. Studies have found different things.

Why studies disagree

Studies of the same milestone don’t always find the same ages. Some common reasons:

• They studied children in different countries or at different times.
• Some asked parents, and some had researchers watch the children.
• They counted different things. For example, does “mama” count as a first word if it’s said to everyone?
• Some studied a few dozen children, and others studied thousands.

When studies disagree, we show both findings and explain the difference. We never quietly average them.

How we choose and check studies

For each milestone, we look for three kinds of study:

• the classic study that first described the milestone or how to measure it;
• the best source for the ages, usually a large study across several countries, or one that pulls together lots of studies;
• any important study that questioned earlier findings.

We confirm every study exists in public research databases. Then we check every number on the page against the original paper, and note exactly where in the paper it comes from. A page only appears here once every study and every number on it has been checked.

We describe each study in our own words, and we never copy its text, tables or charts.

Keeping pages up to date

Each page shows when it was last checked. When new studies change the picture, we update the page.

What this library doesn’t cover

We don’t cover growth (height, weight and head size) or vision and hearing. Health professionals measure these with growth charts and checks.

This library describes what studies found about groups of children. It can’t tell you about your own child, and it never asks about them.

If you’re worried about your child, talk to your child and family health nurse or GP.