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Chapter 8: Estimating with Confidence Section 8.3 Estimating a Population Mean

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To calculate a 95% confidence interval for µ, we use the familiar formula: estimate ± (critical value) (standard deviation of statistic) Choose an SRS of size n from a population having unknown mean µ and known standard deviation σ. As long as the Normal and Independent conditions are met, a level C confidence interval for µ is The critical value z* is found from the standard Normal distribution. One-Sample z Interval for a Population Mean The One-Sample z Interval for a Population Mean In Section 8.1, we estimated the “mystery mean” µ by constructing a confidence interval using the sample mean = 240.79.

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To determine the sample size n that will yield a level C confidence interval for a population mean with a specified margin of error ME: Get a reasonable value for the population standard deviation σ from an earlier or pilot study. Find the critical value z* from a standard Normal curve for confidence level C. Set the expression for the margin of error to be less than or equal to ME and solve for n: Choosing Sample Size for a Desired Margin of Error When Estimating µ We determine a sample size for a desired margin of error when estimating a mean in much the same way we did when estimating a proportion. Choosing Sample Size for a Desired Margin of Error When Estimating µ The margin of error ME of the confidence interval for the population mean µ is

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Example 1: Researchers would like to estimate the mean cholesterol level μ of a particular variety of monkey that is often used in laboratory experiments. They would like their estimate to be within 1 milligram per deciliter (mg/dl) of the true value of μ at a 95% confidence level. A previous study involving this variety of monkey suggests that the standard deviation of cholesterol level is about 5 mg/dl. Obtaining monkeys is time-consuming and expensive, so the researchers want to know the minimum number of monkeys they will need to generate a satisfactory estimate. The critical value for 95% confidence is z* = 1.96. We will use σ = 5 as our best guess for the standard deviation. We round up to 97 monkeys to ensure the margin of error is no more than 1 mg/dl at 95% confidence.

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When we don’t know σ, we can estimate it using the sample standard deviation s x. What happens when we standardize? This new statistic does not have a Normal distribution! When σ is Unknown: The t Distributions

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However, there is a different t distribution for each sample size, specified by its degrees of freedom (df). When we standardize based on the sample standard deviation s x, our statistic has a new distribution called a t distribution. It has a different shape than the standard Normal curve: *It is symmetric with a single peak at 0, *However, it has much more area in the tails.

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Draw an SRS of size n from a large population that has a Normal distribution with mean µ and standard deviation σ. The statistic has the t distribution with degrees of freedom df = n – 1. The statistic will have approximately a t n – 1 distribution as long as the sampling distribution is close to Normal. The t Distributions; Degrees of Freedom When we perform inference about a population mean µ using a t distribution, the appropriate degrees of freedom are found by subtracting 1 from the sample size n, making df = n – 1. We will write the t distribution with n – 1 degrees of freedom as t n – 1.

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When comparing the density curves of the standard Normal distribution and t distributions, several facts are apparent: The density curves of the t distributions are similar in shape to the standard Normal curve. They are symmetric about 0, single- peaked, and bell-shaped. The spread of the t distributions is a bit greater than that of the standard Normal distribution. The t distributions have more probability in the tails and less in the center than does the standard Normal. As the degrees of freedom increase, the t density curve approaches the standard Normal curve ever more closely. We can use Table B in the back of the book to determine critical values t* for t distributions with different degrees of freedom.

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In Table B, we consult the row corresponding to df = n – 1 = 11. The desired critical value is t * = 2.201. We move across that row to the entry that is directly above 95% confidence level. Upper-tail probability p df.05.025.02.01 101.8122.2282.3592.764 111.7962.2012.3282.718 121.7822.1792.3032.681 z*1.6451.9602.0542.326 90%95%96%98% Confidence level C Example 2: Suppose you want to construct a 95% confidence interval for the mean μ of a Normal population based on an SRS of size n = 12. What critical value t* should you use?

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Definition: To construct a confidence interval for µ, Use critical values from the t distribution with n – 1 degrees of freedom in place of the z critical values. That is, Constructing a Confidence Interval for µ

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Choose an SRS of size n from a population having unknown mean µ. A level C confidence interval for µ is where t* is the critical value for the t n – 1 distribution. Use this interval only when: (1)the population distribution is Normal or the sample size is large (n ≥ 30), (2)the population is at least 10 times as large as the sample. The One-Sample t Interval for a Population Mean Random: The data come from a random sample of size n from the population of interest or a randomized experiment. Normal: The population has a Normal distribution or the sample size is large (n ≥ 30). Independent: The method for calculating a confidence interval assumes that individual observations are independent. To keep the calculations reasonably accurate when we sample without replacement from a finite population, we should check the 10% condition: verify that the sample size is no more than 1/10 of the population size. Conditions for Inference about a Population Mean One-Sample t Interval for a Population Mean The one-sample t interval for a population mean is similar in both reasoning and computational detail to the one-sample z interval for a population proportion. As before, we have to verify three important conditions before we estimate a population mean.

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Example 3: A manufacturer of high-resolution video terminals must control the tension on the mesh of fine wires that lies behind the surface of the viewing screen. Too much tension will tear the mesh, and too little will allow wrinkles. The tension is measured by an electrical device with output readings in millivolts (mV). Some variation is inherent in the production process. Here are the tension readings from a random sample of 20 screens from a single day’s production: 269.5297.0269.6283.3304.8280.4233.5257.4317.5 327.4264.7307.7310.0343.3328.1342.6338.8340.1 374.6336.1 Construct and interpret a 90% confidence interval for the mean tension μ of all the screens produced on this day.

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PLAN: If the conditions are met, we can use a one-sample t interval to estimate µ. Random: We are told that the data come from a random sample of 20 screens from the population of all screens produced that day. Normal: Since the sample size is small (n < 30), we must check whether it’s reasonable to believe that the population distribution is Normal. Examine the distribution of the sample data. These graphs give no reason to doubt the Normality of the population Independent: We must assume that at least 10(20) = 200 video terminals were produced this day. It is safe to assume that individual video terminals are independent of one another. STATE: We want to estimate the true mean tension µ of all the video terminals produced this day at a 90% confidence level.

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DO: Using our calculator, we find that the mean and standard deviation of the 20 screens in the sample are: Upper-tail probability p df.10.05.025 181.1301.7342.101 191.3281.7292.093 201.3251.7252.086 80%90%95% Confidence level C Since n = 20, we use the t distribution with df = 19 to find the critical value. CONCLUDE: We are 90% confident that the interval from 292.32 to 320.32 mV captures the true mean tension in the entire batch of video terminals produced that day. From Table B, we find t* = 1.729. Therefore, the 90% confidence interval for µ is:

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Example 4: Environmentalists, government officials, and vehicle manufacturers are all interested in studying the auto exhaust emissions produced by motor vehicles. The major pollutants in auto exhaust from gasoline engines are hydrocarbons, carbon monoxide, and nitrogen oxides (NOX). Researchers collected data on the NOX levels (in grams/mile) for a random sample of 40 light-duty engines of the same type. The mean NOX reading was 1.2675 and the standard deviation was 0.3332. a) Construct and interpret a 95% confidence interval for the mean amount of NOX emitted by light-duty engines of this type.

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STATE: We want to estimate the true mean amount μ of NOX emitted by all light-duty engines of this type at a 95% confidence level. PLAN: We should construct a one-sample t interval for μ if the conditions are met. Random: The data come from a “random sample” of 40 engines from the population of all light-duty engines of this type. Normal: We don’t know whether the population distribution of NOX emissions is Normal. Because the sample size, n = 40, is large (at least 30), we should be safe using t procedures. Independent: We must assume that there are at least 10(40) = 400 light-duty engines of this type. It is safe to assume that individual engines are independent of one another.

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DO: CONCLUDE: We are 95% confident that the interval from 1.1599 to 1.3751 grams/mile contains the true mean level of nitrogen oxides emitted by this type of light-duty engine.

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b) The environmental Protection Agency (EPA) sets a limit of 1.0 gram/mile for NOX emissions. Are you convinced that this type of engine has a mean NOX level of 1.0 or less? Use your interval from (a) to support your answer. The confidence interval from (a) tells us that any value from 1.1599 to 1.3751 g/mi is a plausible value of the mean NOX level μ for this type of engine. Since the entire interval exceeds 1.0, it appears that this type of engine violates EPA limits.

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Definition: An inference procedure is called robust if the probability calculations involved in the procedure remain fairly accurate when a condition for using the procedures is violated. Fortunately, the t procedures are quite robust against non-Normality of the population except when outliers or strong skewness are present. Larger samples improve the accuracy of critical values from the t distributions when the population is not Normal. Using t Procedures Wisely The stated confidence level of a one-sample t interval for µ is exactly correct when the population distribution is exactly Normal. No population of real data is exactly Normal. The usefulness of the t procedures in practice therefore depends on how strongly they are affected by lack of Normality

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Sample size less than 15: Use t procedures if the data appear close to Normal (roughly symmetric, single peak, no outliers). If the data are clearly skewed or if outliers are present, do not use t. Sample size at least 15: The t procedures can be used except in the presence of outliers or strong skewness. Large samples: The t procedures can be used even for clearly skewed distributions when the sample is large, roughly n ≥ 30. Using One-Sample t Procedures: The Normal Condition Except in the case of small samples, the condition that the data come from a random sample or randomized experiment is more important than the condition that the population distribution is Normal. Here are practical guidelines for the Normal condition when performing inference about a population mean.

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Example 5: Determine whether we can safely use a one-sample t interval to estimate the population mean in each of the following settings. a) Histogram of the percent of each state’s residents who are at least 65 years of age. No. We have data on the entire population of the 50 states, so formal inference makes no sense. We can calculate the exact mean for the population. There is no uncertainty due to having only a sample from the population, and no need for a confidence interval.

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b) Stemplot of the force required to pull apart 20 pieces of Douglas fir. No. The data are strongly skewed to the left with possible low outliers, so we cannot trust the t procedures for n = 20.

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c) Stemplot of the lengths of 23 specimens of the red variety of the tropical flower Heliconia. Yes. The data are mildly skewed to the right and there are no outliers. We can use the t distributions for such data.

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8. Researchers were interested in comparing two methods for estimating tire wear. The first method used the amount of weight lost by a tire. The second method used the amount of wear in the grooves of the tire. A random sample of 16 tires was obtained. Both methods were used to estimate the total distance traveled by each tire. The table below provides the two estimates (in thousands of miles) for each tire. a) Construct and interpret a 95% confidence interval for the mean difference μ in the estimates from these two methods in the population of tires.

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State: We want to estimateµ, with 95% confidence, the true mean difference in the estimates from these two methods in the population of tires. Plan: One-sample t interval for μ. Random: The data come from a random sample. Normal: The graph of the differences indicates that there is no strong skewness or outliers. Independent: There are at least 160 tires and it is safe to assume that individual tires are independent of one another.

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Do: Conclude: We are 95% confident that the interval from 2.837 to 6.275 thousands of miles captures the true mean difference in measured tire wear. b) Does your interval in part (a) give convincing evidence of a difference in the two methods of estimating tire wear? Justify your answer. Since the entire interval is positive, and we measured weight – groove, we are reasonably sure that the measurement by the weight method is giving larger numbers than the measurement by the groove method, on average.

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