The techniques are graphical and informal. The normal distribution is the most used distribution in statistics.
Normality is important in statistical inference.
The q th sample quantile is a value along the measurement scale with a proportion q or less of the data less than the q th quantile and a proportion 1-q or less greater than the q th quantile. Because of the discreteness of the data, the proportion of the data less than the q th quantile typically will not be exactly q . Important special cases are the quartiles and the median. The median is the second quartile and has about one-half of the data below it. These definitions are discussed more fully in the Quantiles module.