Mean is one of the measures of central tendency in statistics. The mean or the arithmetic mean is the arithmetic average of a data set or list of numbers. To find the mean, add up all the numbers in a set of data and divide that number by the total number of values in the data set. This will give you the average value. Other types of means include the geometric mean, the harmonic mean, and the Pythagorean mean, which are different statistical measures that have slightly different strengths in calculating mean value.

For the numbers 12, 12, 13, 14, 14, 14, 17, 18, and 21, the mean is the sum of the values (135) divided by the total number of values in the data set (9), which equals 15.

The median is another measure of central tendency that measures where the center of a data set lies. The median is the center or middle number, or the number that is located in the middle of a data set. You find this by arranging your set of numbers in numerical order from the lowest value or smallest number to the largest or highest number and then locating the middle value. If there isn’t an exact middle or midpoint and you have two numbers comprising the middle of the data set, the median number is the average of those two values.

For the numbers 12, 12, 13, 14, 14, 14, 17, 18, and 21, the median is 14.

The mode is the value in a set of data points that occurs most frequently. The mode can be two or more values if there is a tie for the number that occurs most in the frequency distribution. To find this, list your values in ascending order and find the value that shows up at the highest frequency.

For the numbers 12, 12, 13, 14, 14, 14, 17, 18, and 21, the mode is 14.