SOLUTION: A random sample of 20 health maintenance organizations (HMOs) was selected. For each HMO, the co-payment (in dollars) for a doctor's office visit was recorded. The results are as f

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Question 323894: A random sample of 20 health maintenance organizations (HMOs) was selected. For each HMO, the co-payment (in dollars) for a doctor's office visit was recorded. The results are as follows: 11,8,9,12,8,6,9,6,6,7,9,5,11,8,5,10,7,7,5,12
Under the assumption that co-payment amounts are normally distributed, find a 90% confidence interval for the mean co-payment amount in dollars. Then complete the table below.
What is the lower limit of the confidence interval?
What is the upper limit of the confidence interval?
Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
xave=8.05
s=2.282
Sample size is less than 30 (n=20).
Sample standard deviation is known (not the population sigma).
Use t distribution instead of z distribution to construct interval.
Degrees of freedom (DOF) equals n-1=20-1=19.
For DOF=19 and 90%, the critical t value is t=1.328
t%2As%2Fsqrt%28n%29=1.328%282.282%29%2Fsqrt%2820%29=0.678
(8.05-0.678,8.05%2B0.678)
(7.37,8.73)