Given a data set of {3, 7, 8, 5, 12, 14, 21, 13, 18}, find the first quartile.

Step 1 ：Step 1: First, we need to sort the data from lowest to highest, we get {3, 5, 7, 8, 12, 13, 14, 18, 21}.

Step 2 ：Step 2: The first quartile, also known as the lower quartile, is the value below which lies the 25 percent of the data. It can be found by using the formula \(Q1 = \frac{1}{4}(n + 1)th\) term, where n is the total number of data.

Step 3 ：Step 3: Substituting n = 9 into the formula, we get \(Q1 = \frac{1}{4}(9 + 1) = 2.5th\) term.

Step 4 ：Step 4: As we have a fractional number, we have to take the average of the 2nd and 3rd term. The 2nd term in the data set is 5 and the 3rd term is 7.

Step 5 ：Step 5: The first quartile is calculated as \(Q1 = \frac{5 + 7}{2} = 6\).