Question

To study bonding between mothers and infants, a researcher places each mother and her infant in a playroom and has the mother leave for 10 minutes. The researcher records crying time in the sample of infants during this time that the mother was not present and finds that crying time is normally distributed with

M = 4

and

SD = 1.1.

Based on the empirical rule, state the range of crying times within 68% of infants cried, 95% of infants cried, and 99.7% of infants cried.

Answer 1

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We use the empirical rule of normal distribution:

68% is covered by 1 deviation away from mean.

This means that the range of crying time is Mean+/- 1*Stdev = 4 +/- 1.1 = 2.9 to 5.1

95% is covered by 2 deviations away from mean

This means that the range of crying time is Mean+/- 2*Stdev = 4 +/- 2*1.1 = 1.8 to 6.2

99.7% is covered by 3 deviations away from mean

This means that the range of crying time is Mean+/- 3*Stdev = 4 +/- 3*1.1 = 0.7 to 7.3