According to an IRS study, it takes an average of 330 minutes for taxpayers to prepare, copy, and electronically file a 1040 tax form. A consumer watchdog agency selects a random sample of 40 taxpayers and finds the standard deviation of the time to prepare, copy, and electronically file form 1040 is 80 minutes.
a) What assumption or assumptions do you need to make about the shape of the population? Give assumption(s):
b) What is the standard error of the mean in this example?
c) What is the likelihood the sample mean is greater than 320 minutes?
d) What is the likelihood the sample mean is between 320 and 350 minutes?
e) What is the likelihood the sample mean is greater than 350 minutes?
This is a step by step method to help explain the method of converting raw scores to standardized z scores.
Probabilities, z scores and distributions
1. Two hundred raffle tickets are sold. Your friend has 5 people in her family who each bought two raffle tickets. What is the probability that someone from her family will win the raffle?
2. Answer the following:
a. What does it mean to say that x = 152 has a standard score of +1.5?
b. What does it mean to say that a particular value of x has a z-score of -2.1?
c. In general, what is the standard score a measure of?
3. Jolie has a time of 45 minutes for doing her statistics homework. If the mean is 38 minutes and the standard deviation is 3, calculate Jolie's z score. Once calculated, interpret your findings in terms of Jolie's performance (HINT: use the normal distribution and the probability that other students performed better or worse. You will want to think about your z score in terms of standard deviation units.)
4. How does the bell-shaped curve for the sampling distribution of sample means for samples of size n = 100 compare to the bell-shaped curve for the sampling distribution of sample means for samples of size n = 60? Think about it in terms of the standard error of the mean.
5. What are the characteristics of the normal distribution? Why is the normal distribution important in statistical analysis? Provide an example of an application of the normal distribution.
6. In your own words describe the standard normal distribution. Explain why it can be used to find probabilities for all normal distributions.
7. Data is collected from a large Midwestern city. The growth of these adolescents over the course of puberty is normally distributed with a mean of 5.26 inches and a standard deviation of 0.50 inches.
a. What percentage of the adolescents in this city grew less than 4.5 inches? (HINT: convert your score of interest to a standard score and draw a graph to represent the information you want to obtain. Use your table to calculate proportions which are easily converted to percentages by multiplying by 100).
b. What percentage of the adolescents in this city grew more than 5.12 inches?
c. A random sample of 100 adolescents is gathered and the mean growth during puberty was 5.12. If another sample of 100 is taken, what is the probability that its sample mean will be greater than 5.12 inches? (HINT: We are now doing inferential statistics. Identify your population mean, your sample mean, the population standard deviation, and N which is sample size)
d. Why is the z-score used in answering (a), (b), and (c)?
e. Why is the formula for z used in (c) different from that used in (a) and (b)?
8. Assume that the population of heights of male college students is approximately normally distributed with mean m of 68 inches and standard deviation s of 3.75 inches. A random sample of 16 heights is obtained.
a. Describe the distribution of x, height of the college student.
b. Find the proportion of male college students whose height is greater than 70 inches.
c. Describe the distribution of , the mean of samples of size 16.
d. Find the mean and standard error of the distribution.
e. Find P ( > 70
f. Find P ( < 67)