Deciles and quartiles are statistical measures used to divide a data set into ten or four equal parts. Deciles separate data into ten parts based on the values that fall at 10%, 20%, 30%, etc. Quartiles separate data into four parts based on the first (Q1), second (Q2 or median), and third (Q3) values that divide the data in half. The interquartile range is calculated as Q3 - Q1 and measures where the middle 50% of values lie.