Chapter 1 What is Probability?

We hear the word “probability” often. Here are just a few quotes from recent online articles which mention probability (or odds).

  • Researchers say the probability of living past 110 is on the rise (CNBC, July 17, 2021)
  • Forecasters now have a model that can predict the probability of rip currents up to six days out. (CNN, July 18, 2021)
  • A Week of negatives increases probability of stock market correction. (Forbes, July 10, 2021)
  • Less than 1% probability that Earth’s energy imbalance increase occurred naturally (Science Daily, July 28, 2021)
  • Basketball Reference’s Hall of Fame probability model already has Giannis at 67.9 percent. (Bleacher Report, July 23, 2021)
  • Study suggests that the rate of global warming increases the probability of extreme temperatures. (NPR, July 29, 2021)
  • We anticipate an above-average probability for major hurricanes making landfall along the continental United States coastline and in the Caribbean. (Weather Channel, June 1, 2021)
  • Scientists fine-tune odds of asteroid Bennu hitting Earth through 2300 with NASA probe’s help (, Sept 3, 2021)

You have some familiarity with the words “probability”, “chance”, “odds”, or “likelihood” from everyday life. But what do we really mean when talk about “probability”?

This chapter provides a brief but non-technical introduction to randomness and probability. Many of the topics introduced in this chapter will be covered in much more detail in later chapters.

Picture generated by entering “probability” into the random art generator at

Figure 1.1: Picture generated by entering “probability” into the random art generator at