Harmonic Mean
Harmonic mean is a type of numerical average that is usually used in situations when the average rate or rate of change needs to be calculated. Harmonic mean is one of the three Pythagorean means. The remaining two are the arithmetic mean and the geometric mean. These three means or averages are very important as they see widespread use in the field of geometry and music.
if we are given a data series or a set of observations then the harmonic mean can be defined as the reciprocal of the average of the reciprocal terms. This implies that the harmonic mean of a particular set of observations is the reciprocal of the arithmetic mean of the reciprocals. In this article, we will take a detailed look at the different aspects of harmonic mean along with the relationship between all these three means (AM, GM, HM).
What is Harmonic Mean?
Harmonic mean is a measure of central tendency. Say we want to determine a single value that can be used the describe the behavior of data around a central value. Then such a value is known as a measure of central tendency. In statistics, there are three measures of central tendency. These are the mean, median, and mode. The mean can be further classified into arithmetic mean, geometric mean, and harmonic mean.
Harmonic Mean Definition
Harmonic mean is a type of Pythagorean mean. When we divide the number of terms in a data series by the sum of all the reciprocal terms we get the harmonic mean. The value of the harmonic mean will always be the lowest as compared to the geometric and arithmetic mean.
Harmonic Mean Example
Suppose we have a sequence given by 1, 3, 5, 7. The difference between each term is 2. This forms an arithmetic progression. To find the harmonic mean, we take the reciprocal of these terms. This is given as 1, 1/3, 1/5, 1/7 (the sequence forms a harmonic progression). Next, we divide the total number of terms (4) by the sum of the terms (1 + 1/3 + 1/5 + 1/7). Thus, the harmonic mean = 2.3864.
Harmonic Mean Formula
If we have a set of observations given by \(x_{1}\), \(x_{2}\), \(x_{3}\)....\(x_{n}\). The reciprocal terms of this data set will be 1/\(x_{1}\), 1/\(x_{2}\), 1/\(x_{3}\)....1/\(x_{n}\). Thus, the harmonic mean formula is given by
HM = \(\frac{n}{\frac{1}{x_{1}}+\frac{1}{x_{2}}+\frac{1}{x_{3}}...+\frac{1}{x_{n}}}\)
Harmonic Mean of Two Numbers
Say we want to find the harmonic mean of any two numbers, a and b, in a data set. Both a and b are nonzero numbers. Thus, using the aforementioned harmonic mean formula we get,
n = 2
HM = \(\frac{2}{\frac{1}{a}+\frac{1}{b}}\)
HM = \(\frac{2ab}{a + b}\)
How to Find Harmonic Mean?
We can follow the steps given below to find the harmonic mean of the terms in a particular observation set.
 Step 1: Take the reciprocal of each term in the given data set.
 Step 2: Count the total number of terms whose harmonic mean has to be determined. This will be n.
 Step 3: Add all the reciprocal terms.
 Step 4: Divide the value obtained in step 2 by the value from step 3. The resultant will give us the harmonic mean of the required number of terms.
Difference Between Geometric Mean and Harmonic Mean
Both harmonic mean and geometric mean are measures of central tendencies. The difference between geometric mean and harmonic mean is given below:
Geometric Mean  Harmonic mean 
When we are given a data set consisting of n number of terms then we can find the geometric mean by multiplying all the terms and taking the n^{th} root.  Given a data set, the harmonic mean can be evaluated by dividing the total number of terms by the sum of the reciprocal terms. 
The value of the geometric mean is always greater than the harmonic mean but lesser than the arithmetic mean.  The value of the harmonic mean is always lesser than the other two means. 
The geometric mean can be thought of as the arithmetic mean with certain log transformations.  The harmonic mean is the arithmetic mean of the data set with certain reciprocal transformations. 
Example: If we are given a sequence 1, 2, 4, 7. n = 4 GM = (1 × 2 × 4 × 7)^{1/4} = 2.735 
Example: If we are given a sequence 1, 2, 4, 7. n = 4 HM = \(\frac{4}{\frac{1}{1}+\frac{1}{2}+\frac{1}{4}+\frac{1}{7}}\) = 2.113 
Harmonic Mean vs Arithmetic Mean
Both types of means fall under the category of Pythagorean means. The table for the difference between harmonic mean vs arithmetic mean is given below.
Harmonic mean  Arithmetic Mean 
To find the harmonic mean we take the reciprocal of the arithmetic mean of the reciprocal terms in that data set.  To calculate the arithmetic mean we take the sum of all the observations in a data set and divide it by the total number of observations. 
The harmonic mean is the lowest value amongst the three means.  The arithmetic mean is the highest value among all three means. 
Harmonic mean cannot be used on a data set consisting of negative or zero rates.  Arithmetic mean can be calculated even if the data set has negative, positive, and zero values. 
Example: 3, 2, 1, 6 n = 4 HM = \(\frac{4}{\frac{1}{3}+\frac{1}{2}+\frac{1}{1}+\frac{1}{6}}\) = 2 
Example: 3, 2, 1, 6 n = 4 AM = (3 + 2 + 1 + 6) / 4 = 3 
Relation Between Arithmetic Mean, Geometric Mean, and Harmonic Mean
The products of the harmonic mean and the arithmetic mean will always be equal to the square of the geometric mean of the given data set. To understand the relationship between the arithmetic mean, geometric mean, and harmonic mean we will take the help of the formulas. Say we have 2 numbers a and b.
n = 2
According to definition
AM = (a + b) / 2.
HM = 2ab / (a + b) or \((ab)\frac{2}{a + b}\)
GM = √(ab).
Now taking the square we get GM^{2} = (ab). Using this value,
HM = GM^{2} . [2 / (a + b)]
HM = GM^{2} / AM.
Thus, we get
GM^{2} = HM × AM.
Also, HM \(\leq\)GM \(\leq\)AM.
Arithmetic mean is used when the data values have the same units. The geometric mean is used when the data set values have differing units. When the values are expressed in rates we use harmonic mean.
Merits and Demerits of Harmonic Mean
Harmonic mean is a mathematical mean that is usually used to find the average of variables when they are expressed as a ratio of different measuring units. Given below are the merits and demerits of harmonic mean:
Merits of Harmonic Mean
The harmonic mean is completely based on observations and is very useful in averaging certain types of rates. Other merits of the harmonic mean are given below.
 As the value of harmonic mean remains fixed thus, it is rigidly defined.
 Even if there is a sample fluctuation, the harmonic mean does not get significantly affected.
 All items of the series are required to determine the harmonic mean.
Demerits of Harmonic Mean
To calculate the harmonic mean, all elements of the series must be known. In case of unknown elements, we cannot determine the harmonic mean. Given below are other demerits of harmonic mean.
 The method to calculate the harmonic mean can be lengthy and complicated.
 If any term of the given series is 0 then the harmonic mean cannot be calculated.
 The extreme values in a series greatly affect the harmonic mean.
Uses of Harmonic Mean
An important property of harmonic mean is that without taking a common denominator it can be used to find multiplicative and divisor relationships between fractions. This can be a very helpful tool in industries such as finance. Given below are some other reallife applications of harmonic mean.
 The patterns in the Fibonacci series can be determined using the harmonic mean.
 Harmonic mean is used in finance when average multiples have to be evaluated.
 It can be used to calculate quantities such as speed. This is because speed is expressed as a ratio of two measuring units such as km/hr.
 It can also be used to find the average of rates as it assigns equal weight to all data points in a sample.
Weighted Harmonic Mean
Weighted harmonic mean is used when we want to find the average of a set of observations such that equal weight is given to each data point. Let \(x_{1}\), \(x_{2}\), \(x_{3}\)....\(x_{n}\) be the set of observations and \(w_{1}\), \(w_{2}\), \(w_{3}\)....\(w_{n}\) be the corresponding weights. Then the formula for weighted harmonic mean is given as follows:
Weighted HM = \(\frac{\sum_{i = 1}^{n}w_{i}}{\sum_{i = 1}^{n}\frac{w_{i}}{x_{i}}}\)
If we have normalized weights then all weights sum up to 1. That is, \(w_{1}\) + \(w_{2}\) + \(w_{3}\) +....+ \(w_{n}\) = 1
Suppose we have a frequency distribution with n items \(x_{1}\), \(x_{2}\), \(x_{3}\)....\(x_{n}\) having corresponding frequencies \(f_{1}\), \(f_{2}\), \(f_{3}\)....\(f_{n}\) then the weighted harmonic mean is give as:
Weighted HM = \(\frac{n}{\sum_{i=1}^{n}\frac{f_{i}}{x_{i}}}\)
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Important Notes on Harmonic Mean
 The harmonic mean is used when we want to find the reciprocal of the average of the reciprocal terms in a series.
 The formula to determine harmonic mean is \(\frac{n}{\frac{1}{x_{1}}+\frac{1}{x_{2}}+\frac{1}{x_{3}}...+\frac{1}{x_{n}}}\).
 The relationship between HM, GM, and AM is GM^{2} = HM × AM.
 Harmonic mean will have the lowest value, geometric mean will have the middle value and arithmetic mean will have the highest value.
Examples on Harmonic Mean

Example 1: Find the harmonic mean of 7 and 9.
Solution: Using the formula we have HM = 2ab / (a + b)
a = 7 and b = 9
Thus, HM = (2 × 7 × 9) / (7 + 9) = 7.875.
Answer: HM = 7.875 
Example 2: Find the harmonic mean of {2, 4, 5, 11, 14)
Solution: n = 5
HM = \(\frac{5}{\frac{1}{2}+\frac{1}{4}+\frac{1}{5}+\frac{1}{11}+\frac{1}{14}}\) = 4.495.
Answer: 4.495 
Example 3: Calculate the harmonic mean if the arithmetic mean = 9.4, and geometric mean = 8.1649.
Solution: We know that GM^{2} = HM × AM. Thus, HM = GM^{2} / AM = 8.1649^{2}/ 9.4 = 7.09.
Answer: Harmonic mean = 7.09.
FAQs on Harmonic Mean
What is Harmonic Mean in Statistics?
When we take the reciprocal of the arithmetic mean of the reciprocal terms in a data set we get the harmonic mean. Furthermore, if there are certain weights associated with each observation then we can calculate the weighted harmonic mean.
What is Harmonic Mean Formula?
The harmonic mean formula is given as \(\frac{n}{\frac{1}{x_{1}}+\frac{1}{x_{2}}+\frac{1}{x_{3}}...+\frac{1}{x_{n}}}\). Similarly the weighted harmonic mean formula is \(\frac{\sum_{i = 1}^{n}w_{i}}{\sum_{i = 1}^{n}\frac{w_{i}}{x_{i}}}\).
What is the Harmonic Mean of a and b?
If we want to find the harmonic mean of two nonzero numbers a and b then we use the general formula with n = 2. Thus, the formula for the harmonic mean of a and b is 2ab/a + b
How to Calculate Harmonic Mean?
We first take the sum of the reciprocals of each term in the given data set. Then we divide the total number of terms (n) in the data set by this value to get the harmonic mean.
What is the Difference between Geometric Mean and Harmonic Mean?
When we have a data set, the geometric mean can be determined by taking the n^{th} root of the product of all the n terms. To find the harmonic mean we divide n by the sum of the reciprocals of the terms. The harmonic mean will always have a lower value than the geometric mean.
Is Harmonic Mean the Reciprocal of Arithmetic Mean?
Harmonic mean is not the reciprocal of the arithmetic mean. Harmonic mean is the reciprocal of the arithmetic mean of the reciprocal terms in a given set of observations.
What are the Merits and Demerits of Harmonic Mean?
The harmonic mean has a rigid value and does not get affected by fluctuations in the sample however if the sample contains a zero term we cannot calculate the harmonic mean. Also, the formula to determine the harmonic mean can result in complex computations.
What are Applications of Harmonic Mean?
Harmonic mean sees widespread use in geometry and music. It is also used to calculate the average of ratios as it equalizes all the data points.
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