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You measure 43 dogs' weights, and find they have a mean weight of 50 ounces. Assume the population standard deviation is 7.5 ounces. Based on this, construct a 99% confidence interval for the true population mean dog weight.[Note: z-test statistic critical value needs to be used since the population standard deviation is given.]Give your answers as decimals, to two places ± ounces

Question

You measure 43 dogs' weights, and find they have a mean weight of 50 ounces. Assume the population standard deviation is 7.5 ounces. Based on this, construct a 99% confidence interval for the true population mean dog weight.[Note: z-test statistic critical value needs to be used since the population standard deviation is given.]Give your answers as decimals, to two places ± ounces

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Solution

To construct a 99% confidence interval for the true population mean dog weight, we will use the z-distribution because the population standard deviation is known. Here are the steps:

  1. Identify the given information:

    • Sample mean (xˉ\bar{x}) = 50 ounces
    • Population standard deviation (σ\sigma) = 7.5 ounces
    • Sample size (nn) = 43
    • Confidence level = 99%
  2. Find the critical value (zα/2_{\alpha/2}) for a 99% confidence interval:

    • For a 99% confidence level, α=10.99=0.01\alpha = 1 - 0.99 = 0.01.
    • α/2=0.01/2=0.005\alpha/2 = 0.01/2 = 0.005.
    • Using a z-table or standard normal distribution table, the critical value for α/2=0.005\alpha/2 = 0.005 is approximately 2.576.
  3. Calculate the standard error of the mean (SE): SE=σn=7.5437.56.561.14 SE = \frac{\sigma}{\sqrt{n}} = \frac{7.5}{\sqrt{43}} \approx \frac{7.5}{6.56} \approx 1.14

  4. Calculate the margin of error (ME): ME=zα/2×SE=2.576×1.142.94 ME = z_{\alpha/2} \times SE = 2.576 \times 1.14 \approx 2.94

  5. Construct the confidence interval: Lower limit=xˉME=502.94=47.06 \text{Lower limit} = \bar{x} - ME = 50 - 2.94 = 47.06 Upper limit=xˉ+ME=50+2.94=52.94 \text{Upper limit} = \bar{x} + ME = 50 + 2.94 = 52.94

Therefore, the 99% confidence interval for the true population mean dog weight is 47.0647.06 to 52.9452.94 ounces.

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