2.2.1 - Graphs: Dotplots and Histograms

Dotplots and histograms are both graphical displays that can be used with one quantitative variable. Below are descriptions for each along with some examples and instructions for constructing each in Minitab Express.

Dotplots Section

dotplot can also be used to display data concerning one interval- or ratio-level variable. Each dot represents one, or more, data points. In the first example below, each dot represents one observation.

Dotplot of age made with Minitab Express

In this second example, the key at the bottom tells us that each dot may represent up to 2 observations. 

Dotplot of online courses completed made with Minitab Express

 

Later in the course, in Lessons 4 and 5, we will study statistical inference and the application we will use will rely heavily on dotplots. For example, we will use dotplots to determine the proportion of points greater than, less than, or between two values. This can be determined by counting the dots.

MinitabExpress  – Dotplot

This example will use data collected from a sample of students enrolled in online sections of STAT 200 during the Summer 2020 semester. These data can be downloaded as a Minitab Express Project or as a CSV file:

WCStudentData.mpjx

WCStudentData.csv

 

To create a dotplot of the number of online courses completed:

  1. Open the data set in Minitab Express
  2. On a PC or Mac: Select GRAPHS > Dotplot
  3. Select Simple
  4. Double click the variable Online Courses Completed in the box on the left to insert it into the Y variable box on the right
  5. Click OK

This should result in the following dotplot:

Dotplot of online courses completed

Video Walkthrough

Select your operating system below to see a step-by-step guide for this example.

Histograms Section

A histogram displays data concerning one interval- or ratio-level variable. On a histogram, the bars are touching, which is in contrast to the bar charts we saw earlier in this lesson with spaces between their bars.

The height of each bin (i.e., bar) communicates the number of cases with observations in that range. The histograms below both display data concerning the number of online courses completed by a sample of 324 STAT 200 students. The first histogram uses vertical bars; this is the default in most statistical software programs. The second histogram uses horizontal bars. From these plots, we can determine that the range of scores is approximately 0 to 75 online courses completed. The mode, which is the highest bar, is 0 to 5 online courses courses. 

Histogram with vertical bars displaying the number of online courses completed
Histogram with vertical bars
Histogram with horizontal bars displaying the number of online courses completed
Histogram with horizonal bars

MinitabExpress  – Histogram

This example will use data collected from a sample of students enrolled in online sections of STAT 200 during the Summer 2020 semester. These data can be downloaded as a Minitab Express Project or as a CSV file:

WCStudentData.mpjx

WCStudentData.csv

 

To create a histogram of the number of online courses completed in Minitab Express:

  1. Open the data set in Minitab Express
  2. On a PC or Mac: Select GRAPHS > Histogram
  3. Select Simple
  4. Double click the variable Online Courses Completed in the box on the left to insert it into the Y variable box on the right
  5. Click OK

This should result in the following histogram:

Histogram of online courses completed

Video Walkthrough

Select your operating system below to see a step-by-step guide for this example.