## Example 4.15 Creating a One-Way Comparative Histogram

This example illustrates how to create a comparative histogram. The effective channel length (in microns) is measured for 1225 field effect transistors. The channel lengths (Length) are stored in a data set named Channel, which is partially listed in Output 4.15.1:

**
Output 4.15.1
Partial Listing of Data Set Channel**

Lot 1 |
0.91 |

. |
. |

Lot 1 |
1.17 |

Lot 2 |
1.47 |

. |
. |

Lot 2 |
1.39 |

Lot 3 |
2.04 |

. |
. |

Lot 3 |
1.91 |

The following statements request a histogram of Length ignoring the lot source:

title 'Histogram of Length Ignoring Lot Source';
proc univariate data=Channel noprint;
histogram Length;
run;

The resulting histogram is shown in Output 4.15.2.

**
Output 4.15.2
Histogram for Length Ignoring Lot Source**

To investigate whether the peaks (modes) in Output 4.15.2 are related to the lot source, you can create a comparative histogram by using Lot as a classification variable. The following statements create the histogram shown in Output 4.15.3:

title 'Comparative Analysis of Lot Source';
proc univariate data=Channel noprint;
class Lot;
histogram Length / nrows = 3;
run;

The CLASS statement requests comparisons for each level (distinct value) of the classification variable Lot. The HISTOGRAM statement requests a comparative histogram for the variable Length. The NROWS= option specifies the number of rows per panel in the comparative histogram. By default, comparative histograms are displayed in two rows per panel.

**
Output 4.15.3
Comparison by Lot Source**

Output 4.15.3 reveals that the distributions of Length are similarly distributed except for shifts in mean.

A sample program for this example, *uniex09.sas*, is available in the SAS Sample Library for Base SAS software.

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