Question

The third quartile, denoted `Q_3`, is the data value such that what percent of the values are below it?

Answer 1

`75%`

If you work with quartiles, you first order your cases by value.

You then divide your cases in four equal groups.

The value of the case at the border between the first quart and the second is called the first quartile or `Q1`
Between second and third is `Q2` = median
And between third and fourth is `Q3`

So at the `Q3`-point you have passed three-quarters of your values. This is `75%`.

Extra :
With large datasets percentiles are also used (the cases are then divided into 100 groups). If a value is said to be at the `75th` percentile, this means that `75%` of the cases has a lower value.