90th percentile of BMI
The 90th percentile of BMI is the value at which 90% of people have a BMI value below it, or conversely, 10% of people have a BMI value above it. All cohorts had 90th percentile values that were within the obese range of BMI values.
Between 1995 and 2007–08, the 90th percentile of BMI increased significantly for most birth cohorts, excluding the 1923–1932 and 1933–1942 birth cohorts (Figure 1, Table S4).
Between 2007–08 and 2017–18, the 90th percentile of BMI increased significantly for all birth cohorts.
The largest absolute change in the 90th percentile of BMI over the 22 years was among the 1963–1972 birth cohort. For this cohort, the 90th percentile of BMI increased from 31.3 kg/m2 at age 23–32 (in 1995) to 34.3 kg/m2 at age 35–44 (in 2007–08), and increased again to 36.6 kg/m2 at age 45–54 (in 2017–18), corresponding to an increase of 5.3 kg/m2 over the 22 years.
10th percentile of BMI
The 10th percentile of BMI is the value at which 10% of people have a BMI value below it, or conversely, 90% of people have a BMI value above it. All cohorts had 10th percentile values that were within the normal weight range of BMI values (18.50–24.99 kg/m2).
Between 1995 and 2007–08, the 10th percentile of BMI increased slightly for most birth cohorts, except there was no change for the 1933–42 birth cohort and the 1923–1932 birth cohort decreased slightly (Figure 1, Table S4).
Between 2007–08 and 2017–18, the 10th percentile of BMI again increased slightly for almost all birth cohorts, except there was no change for the 1933–1942 birth cohort.
The largest absolute change in the 10th percentile of BMI over the 22 years was among the 1963–1972 birth cohort. For this cohort, the 10th percentile of BMI increased from 20.2 kg/m2 at age 23–32 (in 1995) to 21.3 kg/m2 at age 35–44 (in 2007–08), and increased again to 22.2 kg/m2 at age 45–54 (in 2017–18), corresponding to an increase of 2.0 kg/m2 over the 22 years.
Overall there were much smaller increases in the 10th percentile of BMI within cohorts than there were for the 90th percentile of BMI, which indicates that the top-end of the BMI distribution is increasing faster than the bottom-end.
Box plots of BMI
Box plots give additional insights into changes in the shape of the distribution of BMI over time within birth cohorts and differences between birth cohorts.
The focus of this section is on how the distribution of BMI has changed over time for the 4 birth cohorts that were observed in all 3 of the selected years: 1995, 2007–08 and 2017–18. These birth cohorts range from 1963–1972 (youngest cohort) to 1933–1942 (oldest cohort). Data for the remaining birth cohorts can be shown in Figure 1 for the available survey years by ticking the check-box for each respective cohort.
How to read box plots
The values shown in the box plots below are the 10th percentile, 25th percentile, the median (or 50th percentile), 75th percentile and 90th percentile.
As an example of interpreting the percentiles, the 25th percentile shows at what BMI value 25% of the population has a BMI lower than this, and conversely 75% of the population has a BMI higher than this.
The interquartile range (IQR) is a measure of the variability or spread of the BMI values and is the difference between the 75th percentile (or 3rd quartile) and the 25th percentile (or 1st quartile) values.
