Question: In Case Study, Lee Salk did an experiment to see if hearing the sound of a human heartbeat would help infants gain weight during the first few days of life. By comparing weight gains for two sample groups of infants, he concluded that it did. One group listened to a heartbeat and the other did not.
a. What are the null and alternative hypotheses for this study?
b. Was this a one-sided test or a two-sided test? Explain.
c. What would a type 1 and type 2 error be for this study?
d. Given the conclusion made by Dr. Salk, explain which error he could possibly have committed and which one he could not have committed.
e. Rather than simply knowing whether there was a difference in average weight gains for the two groups, what statistical technique would have provided additional information?
Case Study: Heart or Hypothalamus?
You can learn a lot about nature by observation. You can learn even more by conducting a carefully controlled experiment. This case study has both. It all began when psychologist Lee Salk noticed that despite his knowledge that the hypothalamus plays an important role in emotion, it was the heart that seemed to occupy the thoughts of poets and songwriters. There were no everyday expressions or song titles such as "I love you from the bottom of my hypothalamus" or "My hypothalamus longs for you." Yet, there was no physiological reason for suspecting that the heart should be the center of such attention. Why had it always been the designated choice? Salk began wondering about the role of the heart in human relationships. He also noticed that when on 42 separate occasions he watched a rhesus monkey at the zoo holding her baby, she held the baby on the left side, close to her heart, on 40 of those occasions. He then observed 287 human mothers within 4 days after giving birth and noticed that 237, or 83%, held their babies on the left. Handedness did not explain it; 83% of the right-handed mothers and 78% of the left-handed mothers exhibited the left-side preference. When asked why they chose the left side, the right-handed mothers said it was so their right hand would be free.
The left-handed mothers said it was because they could hold the baby better with their dominant hand. In other words, both groups were able to rationalize holding the baby on the left based on their own preferred hand. Salk wondered if the left side would be favored when carrying something other than a newborn baby. He found a study in which shoppers were observed leaving a supermarket carrying a single bag; exactly half of the 438 adults carried the bag on the left. But when stress was involved, the results were different. Patients at a dentist's office were asked to hold a 5-inch rubber ball while the dentist worked on their teeth. Substantially more than half held the ball on the left. Salk speculated, "It is not in the nature of nature to provide living organisms with biological tendencies unless such tendencies have survival value." He surmised that there must indeed be survival value to having a newborn infant placed close to the sound of its mother's heartbeat. To test this conjecture, Salk designed a study in a baby nursery at a New York City hospital. He arranged for the nursery to have the continuous sound of a human heartbeat played over a loudspeaker. At the end of 4 days, he measured how much weight the babies had gained or lost. Later, with a new group of babies in the nursery, no sound was played. Weight gains were again measured after 4 days. The results confirmed what Salk suspected. Although they did not eat more than the control group, the infants treated to the sound of the heartbeat gained more weight (or lost less). Further, they spent much less time crying. Salk's conclusion was that "newborn infants are soothed by the sound of the normal adult heartbeat." Somehow, mothers intuitively know that it is important to hold their babies on the left side. What had started as a simple observation of nature led to a further understanding of an important biological response of a mother to her newborn infant.