How To Find Class Width in 2022
Data, especially numerical data, can be very useful if you know what to do with it; graphs can be one way to present information or data logically, provided the data allows for that kind of analysis.
It is often of interest to statisticians, instructors, and others to study data distribution. Suppose the data comprises chemistry test results, and you’re interested in the difference between the lowest and highest scores or the fraction of test-takers in various “slots” between them.
The frequency distribution can be a useful tool for scientists, especially (but not only) when the data tend to cluster around a mean or average in the middle of the graph. As the name implies, this is the “bell-shaped curve” that is characteristic of normally distributed data.
What Is a Frequency Distribution?
A frequency distribution is a table that contains intervals of data points, called classes, and the total number of entries for each class. Each class has a frequency f, which is just how many data points it has. Class width refers to the distance between successive classes’ lower (or higher) limits. Each class has two limiting points, the lower-class limit and the upper-class limit. There is no difference between the higher and lower limits of the same class.
Graphs and tables are categorized according to their ranges, which are the sum of the lowest and highest values.
You start using between five and twenty classes when creating a group frequency distribution. The distribution is valid only if the classes have the same width, span, or numerical value. You choose a starting point equal to or less than the lowest value in the entire set once you determine the class width (detailed below).
Class Width Examples
A professor asked students to keep track of their social interactions for a week. As shown in the following grouped frequency distribution, there were 25 social interactions over the week. How do you determine the midpoint of each class?
Class Frequency (f)
- 0–7: 7
- 8–14: 37
- 15–21: 32
- 22–28: 21
- 29–35: 3
It was decided to make the class width seven in this instance. When you combine a range of 35 with an odd number for class width, you get five classes with a range of seven. Four, eleven, eighteen, twenty-five, and thirty-two are the midpoints.
General Guidelines for Determining Classes
If you have a large number of data points, a broad range, or both, choose between five and 20 classes. The following guidelines should also be followed:
- Odd numbers should be used to define the class width. This way, the class midpoints will be integer numbers rather than decimal ones.
- Each data value must belong to exactly one class. No class can include more than one, and none is ignored.
- There must be a continuous class list, so even classes with no entries must be included. There are some exceptions; if you have an empty first or last class, you should exclude it.
- There must be an equal width between the classes, as stated previously. Again, the first and last classes can be exceptions, as they can have values below a certain number at the low end or above a certain number at the high end, etc.
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