Correlation
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Explainer Video
Summary
Correlation
In a nutshell
Bivariate data can be shown on a scatter graph. This helps you understand whether or not data is correlated.
Bivariate data
Bivariate data is the result of observing two variables from the same sample.
Example 1
Regarding a certain population, when registering the height and the weight of each person, you are creating bivariate data: you are relating each person's height with their weight.
Scatter graphs
Bivariate data can be shown on a scatter graph.
The horizontal axis shows the independent variable (the one you control) and the vertical axis shows the dependent variable (the one being measured).
The two variables in a scatter graph are often correlated:
Scatter diagram | Correlation | meaning |
 | Strong positive correlation. | On average, when one variable increases, the other also increases. |
 | Strong negative correlation. | On average, when one variable increases, the other decreases. |
 | No correlation. | There is no clear relation between the two variables. |
Causality
Two variables will have a causal relationship if the change in one variable induces a change in the second.
However, just because two variables are correlated, it doesn't necessarily mean they have a causal relationship. Causation can only be deduced in the context of the data.
Example 2
Describe the correlation between the data shown on the graph. Is there a causal relationship between them?
You can see that when the values increase, values also increase. This can be shown by drawing a line of best fit:
Therefore, the two variables have a strong positive correlation.
However, you cannot state that there's a causal relationship between the variables. You don't know what the axes represent and so the context is not known.
FAQs - Frequently Asked Questions
Does all correlated data have a causal relationship?
No, that can only be determined analysing the context of the question.
How do you know if there's a positive correlation between bivariate data?
Positive correlation occurs when the increase of one variable corresponds to an increase of the other variable.
What are bivariate data?
Bivariate data is data obtained from the observation of two variables of the same sample.