Chapter 5

Sampling and Probability


Samples and Their Populations

  • Random Sampling
  • Convenience Sampling
  • The Problem with a Biased Sample
  • Random Assignment


  • Coincidence and Probability
  • Expected Relative-Frequency Probability
  • Independence and Probability

Inferential Statistics

  • Developing Hypotheses
  • Making a Decision About the Hypothesis

Type I and Type II Errors

  • Type I Errors
  • Type II Errors

Next Steps: The Shocking Prevalence of Type I Errors


  • You should understand the difference between a sample and a population (Chapter 1).
  • You should know how to measure central tendency, especially the mean (Chapter 4).


Lillian and Frank Gilbreth and Their Children Sampling is a way to do things more efficiently, based on principles that Lillian Gilbreth, the “mother of scientific management,” and her husband Frank applied to raising a happy if occasionally chaotic family. Eleven of their twelve children are pictured here.
© Bettmann/CORBIS

A pioneer in the field of organizational and industrial psychology was such an introverted, timid little girl that her parents home-schooled her until she was 9 years old. That home was a busy place, with little Lillian, the eldest of 9 children, often filling in for her ill mother. The hectic pace of life that had begun during Lillian’s childhood kept up for the rest of her life. She and her husband Frank Gilbreth, both of them efficiency experts, created a consulting business. In attendance for the awarding of her PhD in industrial psychology from Brown University were 4 of the couple’s 12 children. The family’s lifestyle inspired 2 of the children to later write the book Cheaper By the Dozen, which also became a popular film.

Lillian Gilbreth continued to support her family as an industrial consultant after Frank died at a relatively young age. All this occurred in an era that allowed only her husband’s name to appear on the books they wrote together, out of fear of losing credibility if publishers advertised a female author! Frank had never earned a degree; Lillian had a master’s degree from the University of California, Berkeley, in addition to her PhD from Brown.

The couple pioneered the use of filming people at work in order to analyze the motions needed to perform a task more efficiently, and applied those same principles to helping their children manage their lives more effectively. Lillian Gilbreth’s ability to think scientifically also helped her create small and large innovations for the home and workplace. Foot pedals on garbage cans? Thank Lillian Gilbreth. The field of scientific management? Thank Lillian and Frank Gilbreth.

The San Diego Supercomputer Center described Lillian Gilbreth as “a genius in the art of living” and an intimidating demonstration of a woman who successfully enjoyed having it all—a large family, a busy and satisfying career, and a successful marriage that was a genuine partnership. The driving idea behind all the efforts of Lillian and Frank Gilbreth was humane efficiency, and the couple applied that same principle to life in their busy household. Efficiency is the driving idea behind sampling, as well. Why work any harder when an easier way is readily available? Why study 400,000 people if 400 people, sampled properly, will yield the same information?