Percentiles are measures of spread which divide the data into 100 equal portions. This is used to analyse the spread of large sets of data like data collected by census. The data is then divided into 100 portions and the report is given as a percentage.

The value at the 5th percentile implies that 5% of values lie below 5th percentile and 95% of the values lie above the 5th percentile.

The value at quartile 1 implies that 25% of the values lie below 25th percentile and 75% of the values lie above the25th percentile.

The concept of the percentiles is used when the data is large. This concept will be used in growth charts. The curve on the growth chart represents the percentile values of the collected data from different age groups .E.g. height, length, weight, circumference of the head.

The growth chart is used to compare the BMI of an individual versus the one of their age group. This is also used to determine the health status of individuals.

3 types of questions should be covered:

- Reading information from the chart
- Understanding significance of the curve
- Understanding significance of positioning on the chart

Khan Academy Medicine. (2013). Growth curves (CC BY-NC)

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