# Histogram

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Template:Infobox quality tool
A **histogram** is a graphical representation of the distribution of data. It is an estimate of the probability distribution of a continuous variable (quantitative variable) and was first introduced by Karl Pearson.^{[1]} To construct a histogram, the first step is to "bin" the range of values -- that is, divide the entire range of values into a series of small intervals -- and then count how many values fall into each interval. A rectangle is drawn with height proportional to the count and width equal to the bin size, so that rectangles about each other. A histogram may also be normalized displaying relative frequencies. It then shows the proportion of cases that fall into each of several categories, with the sum of the heights equaling 1. The bins are usually specified as consecutive, non-overlapping intervals of a variable. The bins (intervals) must be adjacent, and usually equal size.^{[2]} The rectangles of a histogram are drawn so that they touch each other to indicate that the original variable is continuous.^{[3]}

Histograms give a rough sense of the density of the data, and often for density estimation: estimating the probability density function of the underlying variable. The total area of a histogram used for probability density is always normalized to 1. If the length of the intervals on the *x*-axis are all 1, then a histogram is identical to a relative frequency plot.

A histogram can be thought of as a simplistic kernel density estimation, which uses a kernel to smooth frequencies over the bins. This yields a smoother probability density function, which will in general more accurately reflect distribution of the underlying variable. The density estimate could be plotted as an alternative to the histogram, and is usually drawn as a curve rather than a set of boxes.

A variable binwidth histogram was introduced by Denby and Mallows (2009). Examples of this are displayed on Census bureau data below.

Another alternative is the average shifted histogram which is fast to compute, and gets a smooth curve estimate of the density without using kernels.

The histogram is one of the seven basic tools of quality control.^{[4]}

Histograms are often confused with bar charts. A bar chart is a plot of categorical variables, and the discontinuity should be indicated by having gaps between the rectangles. Often this is neglected which may lead to a bar chart being confused for a histogram.

## Etymology

The etymology of the word *histogram* is uncertain. Sometimes it is said to be derived from the Greek *histos* 'anything set upright' (as the masts of a ship, the bar of a loom, or the vertical bars of a histogram); and *gramma* 'drawing, record, writing'. It is also said that Karl Pearson, who introduced the term in 1891, derived the name from "historical diagram".^{[5]}

## Examples

This is a toy example

Bin | Count | |
---|---|---|

-3.5 | 9 | |

-2.5 | 32 | |

-1.5 | 109 | |

-0.5 | 180 | |

0.5 | 132 | |

1.5 | 34 | |

2.5 | 4 | |

3.5 | 9 |

The language used to describe the patterns in a histogram are symmetric, skewed left or right, unimodal, bimodal or multimodal.

- Symmetric2.png
Symmetric

It is a good idea to plot your data on several different binwidths to learn more about it. Here is an example on tips given in a restaurant.

Here are a couple more examples.

Prices of houses sold in Ames in 2009, exhibits some right-skew.

Aces by players in a grand slam tennis tournament, facetted by gender. There are more aces in the mens game.

The U.S. Census Bureau found that there were 124 million people who work outside of their homes.^{[6]} Using their data on the time occupied by travel to work, Table 2 below shows the absolute number of people who responded with travel times "at least 30 but less than 35 minutes" is higher than the numbers for the categories above and below it. This is likely due to people rounding their reported journey time.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B=
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Data by absolute numbers Interval Width Quantity Quantity/width 0 5 4180 836 5 5 13687 2737 10 5 18618 3723 15 5 19634 3926 20 5 17981 3596 25 5 7190 1438 30 5 16369 3273 35 5 3212 642 40 5 4122 824 45 15 9200 613 60 30 6461 215 90 60 3435 57

This histogram shows the number of cases per unit interval as the height of each block, so that the area of each block is equal to the number of people in the survey who fall into its category. The area under the curve represents the total number of cases (124 million). This type of histogram shows absolute numbers, with Q in thousands.

Data by proportion Interval Width Quantity (Q) Q/total/width 0 5 4180 0.0067 5 5 13687 0.0221 10 5 18618 0.0300 15 5 19634 0.0316 20 5 17981 0.0290 25 5 7190 0.0116 30 5 16369 0.0264 35 5 3212 0.0052 40 5 4122 0.0066 45 15 9200 0.0049 60 30 6461 0.0017 90 60 3435 0.0005

This histogram differs from the first only in the vertical scale. The area of each block is the fraction of the total that each category represents, and the total area of all the bars is equal to 1 (the fraction meaning "all"). The curve displayed is a simple density estimate. This version shows proportions, and is also known as a unit area histogram.

In other words, a histogram represents a frequency distribution by means of rectangles whose widths represent class intervals and whose areas are proportional to the corresponding frequencies: the height of each is the average frequency density for the interval. The intervals are placed together in order to show that the data represented by the histogram, while exclusive, is also contiguous. (E.g., in a histogram it is possible to have two connecting intervals of 10.5–20.5 and 20.5–33.5, but not two connecting intervals of 10.5–20.5 and 22.5–32.5. Empty intervals are represented as empty and not skipped.)^{[7]}

## Mathematical definition

In a more general mathematical sense, a histogram is a function *m*_{i} that counts the number of observations that fall into each of the disjoint categories (known as *bins*), whereas the graph of a histogram is merely one way to represent a histogram. Thus, if we let *n* be the total number of observations and *k* be the total number of bins, the histogram *m*_{i} meets the following conditions:

### Cumulative histogram

A cumulative histogram is a mapping that counts the cumulative number of observations in all of the bins up to the specified bin. That is, the cumulative histogram *M*_{i} of a histogram *m*_{j} is defined as:

### Number of bins and width

There is no "best" number of bins, and different bin sizes can reveal different features of the data. Grouping data is at least as old as Graunt's work in the 17th century, but no systematic guidelines were given^{[8]} until Sturges's work in 1926.^{[9]}

Using wider bins where the density is low reduces noise due to sampling randomness; using narrower bins where the density is high (so the signal drowns the noise) gives greater precision to the density estimation. Thus varying the bin-width within a histogram can be beneficial. Nonetheless, equal-width bins are widely used.

Some theoreticians have attempted to determine an optimal number of bins, but these methods generally make strong assumptions about the shape of the distribution. Depending on the actual data distribution and the goals of the analysis, different bin widths may be appropriate, so experimentation is usually needed to determine an appropriate width. There are, however, various useful guidelines and rules of thumb.^{[10]}

The number of bins *k* can be assigned directly or can be calculated from a suggested bin width *h* as:

The braces indicate the ceiling function.

- Square-root choice

which takes the square root of the number of data points in the sample (used by Excel histograms and many others).^{[11]}

- Sturges' formula

Sturges' formula^{[9]} is derived from a binomial distribution and implicitly assumes an approximately normal distribution.

It implicitly bases the bin sizes on the range of the data and can perform poorly if *n* < 30, because the number of bins will be small—less than seven—and unlikely to show trends in the data well. It may also perform poorly if the data are not normally distributed.

- Rice Rule

The Rice Rule ^{[12]} is presented as a simple alternative to Sturges's rule.

- Doane's formula

Doane's formula^{[13]} is a modification of Sturges' formula which attempts to improve its performance with non-normal data.

where is the estimated 3rd-moment-skewness of the distribution and

- Scott's normal reference rule

where is the sample standard deviation. Scott's normal reference rule^{[14]} is optimal for random samples of normally distributed data, in the sense that it minimizes the integrated mean squared error of the density estimate.^{[8]}

- Freedman–Diaconis' choice

The Freedman–Diaconis rule is:^{[15]}^{[8]}

which is based on the interquartile range, denoted by IQR. It replaces 3.5σ of Scott's rule with 2 IQR, which is less sensitive than the standard deviation to outliers in data.

- Choice based on minimization of an estimated
*L*^{2}^{[16]}risk function

where and are mean and biased variance of a histogram with bin-width , and .

- Remark

A good reason why the number of bins should be proportional to is the following: suppose that
the data are obtained as independent realizations of a bounded probability distribution with smooth density.
Then the histogram remains equally »rugged« as tends to infinity. If is the »width« of the
distribution (e. g., the standard deviation or the inter-quartile range), then the number of units in a bin (the frequency) is
of order and the *relative* standard error is of order .
Comparing to the next bin, the relative change of the frequency is of order provided that the
derivative of the density is non-zero. These two are of the same order if is of order ,
so that is of order .

This simple cubic root choice can also be applied to bins with non-constant width.

## See also

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- Data binning
- Density estimation
- Kernel density estimation, a smoother but more complex method of density estimation

- Freedman–Diaconis rule
- Image histogram
- Pareto chart
- Seven Basic Tools of Quality
- V-optimal histograms

## References

- ↑ Template:Cite doi
- ↑ Howitt, D. and Cramer, D. (2008)
*Statistics in Psychology*. Prentice Hall - ↑ Charles Stangor (2011) "Research Methods For The Behavioral Sciences". Wadsworth, Cengage Learning. ISBN 9780840031976.
- ↑ Template:Cite web
- ↑ {{#invoke:Citation/CS1|citation |CitationClass=journal }}
- ↑ US 2000 census.
- ↑ Dean, S., & Illowsky, B. (2009, February 19). Descriptive Statistics: Histogram. Retrieved from the Connexions Web site: http://cnx.org/content/m16298/1.11/
- ↑
^{8.0}^{8.1}^{8.2}{{#invoke:citation/CS1|citation |CitationClass=book }} - ↑
^{9.0}^{9.1}{{#invoke:Citation/CS1|citation |CitationClass=journal }} - ↑
*e.g.*§ 5.6 "Density Estimation", W. N. Venables and B. D. Ripley,*Modern Applied Statistics with S*(2002), Springer, 4th edition. ISBN 0-387-95457-0. - ↑ EXCEL 2007: Histogram
- ↑ Online Statistics Education: A Multimedia Course of Study (http://onlinestatbook.com/). Project Leader: David M. Lane, Rice University (chapter 2 "Graphing Distributions", section "Histograms")
- ↑ Doane DP (1976) Aesthetic frequency classiﬁcation. American Statistician, 30: 181–183
- ↑ {{#invoke:Citation/CS1|citation |CitationClass=journal }}
- ↑ {{#invoke:Citation/CS1|citation |CitationClass=journal }}
- ↑ {{#invoke:Citation/CS1|citation |CitationClass=journal }}

## Further reading

- Lancaster, H.O.
*An Introduction to Medical Statistics.*John Wiley and Sons. 1974. ISBN 0-471-51250-8

## External links

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- Journey To Work and Place Of Work
*(location of census document cited in example)* - Smooth histogram for signals and images from a few samples
- Histograms: Construction, Analysis and Understanding with external links and an application to particle Physics.
- A Method for Selecting the Bin Size of a Histogram
- Interactive histogram generator
- Matlab function to plot nice histograms
- Dynamic Histogram in MS Excel
- Histogram construction and manipulation using Java applets, and charts on SOCR