Anonymous asked in Science & MathematicsMathematics · 1 decade ago

Question Questions?

Find the area under the standard normal curve between z = 1.6 and z = 2.6.





A business wants to estimate the true mean annual income of its customers. It randomly samples 220 of its customers. The mean annual income was $61,400 with a standard deviation of $2,200. Find a 95% confidence interval for the true mean annual income of the business’ customers.

IQ test scores are normally distributed with a mean of 100 and a standard deviation of 15. An individual's IQ score is found to be 120. Find the z-score corresponding to this value.





2 Answers

  • 1 decade ago
    Favorite Answer

    a) P(1.6<Z<2.6)=P[Z<2.6]-P[Z<1.6]



    b) n=220




    First of all, we know the distribution is normal, since the sample size is large (>30). That means our test statistic is either Z or t. Since the standard deviation was estimated from a sample, the population standard deviation is unknown. This means we will use the t-statistic.

    So, the confidence interval will be of the form:

    mean +/- t(1-alpha/2;n-1) x s/sqrt(n)

    ie. 61400 +/- t(0.975;219) x 2200/sqrt(220)

    ie. 61400 +/- 0.8347 x 148.324

    ie. 61400 +/- 124

    ie. (61276,61524)

    c) Z=(x-mu)/sigma




  • 4 years ago

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