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In statistics, a histogram is a graphical display of tabulated frequencies, shown as bars. It shows what proportion of cases fall into each of several categories. A histogram differs from a bar chart in that it is the area of the bar that denotes the value, not the height, a crucial distinction when the categories are not of uniform width (Lancaster, 1974). The categories are usually specified as non-overlapping intervals of some variable. The categories (bars) must be adjacent.

The word histogram is derived from Greek: histos 'anything set upright' (as the masts of a ship, the bar of a loom, or the vertical bars of a histogram); gramma 'drawing, record, writing'. The histogram is one of the seven basic tools of quality control, which also include the Pareto chart, check sheet, control chart, cause-and-effect diagram, flowchart, and scatter diagram. A generalization of the histogram is kernel smoothing techniques. This will construct a very smooth probability density function from the supplied data.

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 so that the height of each bar is equal to the proportion of total people in the survey who fall into that category. The area under the curve represents the total number of cases (124 million). This type of histogram shows absolute numbers.

Interval | Width | Quantity (Q) | Q/total/width |
---|---|---|---|

0 | 5 | 4180 | 0.0067 |

5 | 5 | 13687 | 0.0220 |

10 | 5 | 18618 | 0.0300 |

15 | 5 | 19634 | 0.0316 |

20 | 5 | 17981 | 0.0289 |

25 | 5 | 7190 | 0.0115 |

30 | 5 | 16369 | 0.0263 |

35 | 5 | 3212 | 0.0051 |

40 | 5 | 4122 | 0.0066 |

45 | 15 | 9200 | 0.0049 |

60 | 30 | 6461 | 0.0017 |

90 | 60 | 3435 | 0.0004 |

This histogram differs from the first only in the vertical scale. The height of each bar is the decimal percentage of the total that each category represents, and the total area of all the bars is equal to 1, the decimal equivalent of 100%. 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. They only place the bars together to make it easier to compare data.

In a more general mathematical sense, a histogram is a mapping $m\_i$ that counts the number of observations that fall into various 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:

$n\; =\; sum\_\{i=1\}^k\{m\_i\}.$

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 $m\_i$ is defined as:

$M\_i\; =\; sum\_\{j=1\}^i\{m\_j\}$

The number of bins $k$ can be calculated directly, or from a suggested bin width $h$:

- $k\; =\; left\; lceil\; frac\{max\; x\; -\; min\; x\}\{h\}\; right\; rceil.$

- $k\; =\; lceil\; log\_2\; n\; +\; 1\; rceil$

- $h\; =\; frac\{3.5\; s\}\{n^\{1/3\}\}$

- $h\; =\; 2\; frac\{operatorname\{IQR\}(x)\}\{n^\{1/3\}\}$

- $h(f)(y)\; =\; sum\_\{xiin\{x\; :\; f(x)=y\}\}\; frac\{1\}$
>.

- Density estimation
- Freedman-Diaconis rule
- Image histogram
- Kernel density estimation , another method of visualizing probability density functions that can be preferred to histograms.

- Webster's Third New International Dictionary, Merriam-Webster; Ind Una edition (June 2002).
- Lancaster, H.O. An Introduction to Medical Statistics. John Wiley and Sons. 1974. ISBN 0 471 51250-8

- Journey To Work and Place Of Work (location of census document cited in example)
- Understanding histograms in digital photography
- 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

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Last updated on Tuesday October 07, 2008 at 06:49:53 PDT (GMT -0700)

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This article is licensed under the GNU Free Documentation License.

Last updated on Tuesday October 07, 2008 at 06:49:53 PDT (GMT -0700)

View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation

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