How do you know when to use a bar chart and when to use a pie chart in statistics?

Pie charts are to show a cut of the whole, the idea being that the whole pie equals 100% and each slice is a statistical probability. Bar charts are excellent for showing test data and ongoing procedures.

Thank you.

Also bar charts do not have to add up to 100%, they can represent overlap in the data.

I though Pie Chart is used for expences Accounts, outgoing expendature, revenue. & Bar Charts for incoming revenue generated. I will stand corrected on this!