The main step in constructing a Q-Q plot is calculating or estimating the quantiles to be plotted.

If the Q-Q plot is based on data, there are multiple quantile estimators in use.

In that case, to make the Q-Q plot, one orders each set in increasing order, then pairs off and plots the corresponding values.

The points plotted in a Q-Q plot are always non-decreasing when viewed from left to right.

For example, it is not possible to determine the median of either of the two distributions being compared by inspecting the Q-Q plot.

The distance between medians is another measure of relative location reflected in a Q-Q plot.

Further it gives the option to see the Q-Q plot in terms of calculated and observed cumulative frequencies.

It can be interpreted via a Q-Q plot.

Both the histogram and the Q-Q plot show that the distribution of t -statistics is not normal.

They correspond to the same genes identified as outliers in the Q-Q plot (Figure 2).