Exactly 25% of your data is less than the first quartile (Q1, also called the 25th percentile). It equals the data value at position (N + 1) / 4. If this position number is not an integer, Minitab extrapolates between the two observations on either side of that position.

Exactly 75% of your data is less than the third quartile (Q3, also called the 75th percentile). It equals the data value at position 3(N + 1) / 4. If this position number is not an integer, Minitab extrapolates between the two observations on either side of that position.

Q1 and Q3 are often used to calculate the interquartile range (IQR), which is another statistic used to describe dispersion. The IQR is the range of the middle 50% of the values and is calculated by the formula Q3 - Q1. The IQR is relatively insensitive to extreme values.

The first and third quartiles are listed in the middle of the Graphical Summary.

For the precipitation data, Q1 is 2, Q3 is 4, and the IQR is 4 - 2 = 2.

Notice the IQR would remain the same even if there were 30 days with rain in April instead of 10.