Creating Cumulative Frequency Tables
Building a cumulative frequency table from a regular frequency table is like climbing stairs – each step adds to what you've already climbed. You're essentially creating a running total that shows how many items fall within each boundary.
The process is surprisingly simple: start with your first frequency value, then keep adding each subsequent frequency to your running total. For the cat weights example, you go from 8, to 8+10=18, to 18+21=39, and so on until you reach your final total of 80 cats.
Cumulative frequency graphs are plotted using the upper boundary of each class interval on the x-axis. The points are then connected with smooth curves, creating your visual tool for reading off quartiles and other statistical measures.
Remember: The interquartile range is found by subtracting Q1 (25% position) from Q3 (75% position), giving you a measure of how spread out the middle 50% of your data is.