Frequency Distributions
A Frequency Distribution is a listing of observed values and corresponding frequency of each value.
Do problem 9, page 848
The data is generally divided into a number of classes.
- The classes must be the same width.
- The classes must not overlap.
- Each piece of data should belong to exactly one class.
Guidelines
- Too many or too few classes yield a useless diagram.
- 5-12 classes is a guideline, but it depends on the data.
- Calculate the Class width. (lower limit - previous lower limit)
- Then calculate the classes.
If the class width is 7, and the first data is at 2 and the last data is 31. The classes would be:
- 2 to 8
- 9 to 15
- 16 to 22
- 23 to 29
- 30 to 36
2,9,16,23 and 30 are the lower class limits
8, 15, 22, 29 and 36 are the upper class limits.
The modal class is the class with the greatest frequency.
The midpoint of a class or class mark is found by
lower limit + upper limit
mark = -------------------------
2
Statistical Graphs
- Circle graphs
- Calculate the frequency distribution.
- For each class, calculate %
- For each class, multiply % by 360 to give angle.
- Draw graph, and label things.
- Histograms and Frequency polygons.
- Calculate class mark.
- For a histogram draw a rectangle
- For a frequency polygon, plot a point.
- For a frequency polygon, connect the dots.
- Stem and Leaf Displays
- A quick and dirty histogram
- Do number 24, page 859