# Concordance Rate: Definition & Calculation

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• 0:05 Definition of Concordance Rate
• 1:06 Concordance Rate and…
• 2:02 Calculating Pair-Wise…
• 4:20 How to Interpret…
• 5:25 Mental Illness…
• 6:16 Lesson Summary
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Lesson Transcript
Instructor: Paul Jeffries

Paul has taught psychology for over 10 years.

In this lesson, you will learn what a concordance rate is and how to calculate it. You will also learn how concordance rates have been used to understand the causes of mental illness.

## Definition of Concordance Rate

What causes some people to develop mental illnesses such as depression, bipolar disorder, and schizophrenia? Is it nature or nurture? Genes or environment? The answer turns out to be both. Most, if not all, mental illnesses are caused by an interaction of genes and environment. Since genes and environment both matter, the question is no longer 'nature or nurture?' but rather 'what is the relative importance of nature and nurture?'

Concordance rate is a statistical index that researchers use to determine the relative influence of nature and nurture. The word concordance means 'to agree,' so the concordance rate is the rate of agreement. Formally, the concordance rate is the percent of cases in which both members of a pair have a particular attribute. In a study of the relative influence of genes and environment on a mental illness, for example, the attribute is the particular mental illness, and the pairs studied are usually twins.

## Concordance Rate and Twin Studies

Identical twins result when a single fertilized egg splits into two eggs, and each egg develops into an embryo. Fraternal twins result when two separate eggs are fertilized, and each egg develops into an embryo. Since identical twins result from a single fertilized egg, they share 100% of their genes. In contrast, fraternal twins share only 50% of their genes on average. Fraternal twins are no more genetically similar than non-twin siblings are.

Fraternal twins that are raised together will share the same environment and 50% of their genes on average. Identical twins that are raised together will share the same environment and 100% of their genes. So, if identical twins are more similar to each other on a given attribute, it must be because of their greater genetic similarity. Genes, therefore, must play a role in determining that attribute.

## Calculating the Pair-Wise Concordance Rate

Here is a simple, hypothetical example to show you how concordance rates are calculated and used. Suppose that we are interested in knowing the relative influence of genes and environment in causing the disease morbus. This is a made up disease, but at this point, we want to focus on the reasoning behind concordance rate studies rather than any particular disease.

For our study, we recruited 100 pairs of identical twins and 100 pairs of fraternal twins. We picked the twin pairs so that at least one twin in each pair has the disease morbus. If only one twin in a pair has morbus, the twins are said to be discordant for morbus. If both twins in a pair have the disease, they are said to be concordant for morbus.

This table shows the number of concordant and discordant pairs among our identical and fraternal twins.

Number of Concordant Pairs Number of Discordant Pairs Total Number of Pairs
Identical Twins 50 50 100
Fraternal Twins 17 83 100

We will calculate two concordance rates: one for identical twins and the other for fraternal twins. There are a couple of ways to calculate the concordance rate. The simplest way, described here, is called the pairwise concordance rate. The formula for the pairwise concordance rate is this:

First, we will calculate the pairwise concordance rate for identical twins. We plug into the equation the number of concordant identical twin pairs (50) and the number of discordant identical twin pairs (50). (Note that C I is used to indicate it is the rate for identical twins.)

Next, we will calculate the pairwise concordance rate for fraternal twins. This time we put into our equation the number of concordant fraternal twin pairs (17) and the number of discordant fraternal twin pairs (83). (C F is used to indicate it is the rate for fraternal twins.)

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