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Violin plot

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A violin plot is a specialized statistical graphic used for comparing probability distributions. It is a tool that helps researchers visualize how data is spread out across different groups. While it shares many features with the box plot, it provides much more detailed information. This extra detail comes from adding a rotated kernel density plot to each side of the graphic.

Violin plot
Violin plot

By looking at the shape of the plot, a reader can see the full distribution of the data. This makes it much more descriptive than a simple summary of numbers.

To understand how a violin plot works, you must look at its various internal layers. The most basic version includes all the data found in a standard box plot. This includes a marker for the median, which is the middle value of the dataset. It also includes a box or a marker to show the interquartile range. In cases where the number of samples is low, the plot might even show every individual sample point.

Violinplot-hiv-paper-plot-pathogens
Violinplot-hiv-paper-plot-pathogens

Beyond these elements, the outer shape represents the probability density of the data. This density is usually smoothed out by a mathematical tool called a kernel density estimator.

These plots can be organized into several distinct layers of information. The outermost layer of the shape represents all possible results within the dataset. A second layer inside might represent the specific values that occur 95% of the time. If the plot includes a third layer, it might show the values that occur 50% of the time. This layering allows a scientist to see both the broad possibilities and the most common outcomes at once. This structure helps differentiate the violin plot from a basic kernel density plot.

One of the most significant advantages of this tool is its ability to show multimodal data. Multimodal data is information that contains more than one peak. In a standard box plot, these separate peaks might be hidden within a single summary. However, a violin plot clearly displays the presence of different peaks. It shows exactly where these peaks are located along the axis. It also shows their relative amplitude, or how tall each peak is compared to the others.

Violin plot
Violin plot

The history of this graphic is tied to the evolution of statistical visualization. In 1977, a mathematician named John Tukey created the box plot. For twenty years, the box plot was a standard way to show data summaries. Then, in 1997, Jerry L. Hintze and Ray D. Nelson proposed the violin plot. They wanted a way to display even more information than the box plot allowed. The name "violin plot" comes from the way the rounded density shapes resemble the body of a violin.

Violinplot-hiv-paper-plot-pathogens
Violinplot-hiv-paper-plot-pathogens

Some people also refer to these graphics as bean plots.

Researchers use violin plots to compare variables across different categories. For example, a scientist might use one to compare temperature distributions between day and night. Another researcher might use them to compare car prices across different car makers. These comparisons help identify how different groups behave. While they are powerful, violin plots are currently less popular than box plots. This is often because they can be harder for uninitiated readers to interpret correctly.

Violin plot
Violin plot

If a reader finds them too complex, they might use stacked histograms or kernel density plots instead.

It is important to note that the definition of a violin plot has shifted slightly over time. Originally, the term meant a combination of a box plot and a two-sided kernel density plot. Today, some people use the term to describe only the two-sided kernel density plots. They may leave out the box plot elements entirely. Despite these changes in usage, the core purpose remains the same. The goal is to provide a visual map of how likely different values are to occur within a population.

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File:Violin plot.gif
Violin plot.gif
File:Violinplot-hiv-paper-plot-pathogens.svg
Violinplot-hiv-paper-plot-pathogens.svg
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