ObjectivesBy the end of this lesson, you will be able to... Show
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The last parameters we need to find confidence intervals for are the population variance (σ2) and standard deviation (σ). There are many instances where we might be interested in knowing something about the spread of a population based on a sample. For example, we might believe that a particular group of students seems to have a wider variation in their grades than those from the past. Or a part manufacturer may be concerned that one of the parts it's manufacturing is too inconsistent, even though the mean may be at specifications. Before we can develop a confidence interval for the variance, we need another distribution. The Chi-Square (Χ2) distributionNote: "chi-square" is pronounced "kai" as in sky, not "chai" like the tea. The Chi-Square (Χ2) distributionIf a simple random sample size n is obtained from a normally distributed population with mean μ and standard deviation σ, then has a chi-square distribution with n-1 degrees of freedom. Properties of the Χ2 distribution
Finding Critical ValuesFind critical values in the Χ2 distribution using a table is done in the same manner we found critical values for the t-distribution. Before we start the section, you need a copy of the table. You can download a printable copy of this table, or use the table in the back of your textbook. It should look something like the image below (trimmed to make it more viewable). Notice that the table is similar to the t-table, with probabilities along the top and critical values in the middle. This is because we primarily use the chi-square-table to find critical values. Let's try an example. Example 1 Find the critical values in the Χ2 distribution which separate the middle 95% from the 2.5% in each tail, assuming there are 12 degrees of freedom. You can use the table above, or print one out yourself. Your textbook should also come with a copy you can use. [ reveal answer ] We can see form the table that the two critical values are 4.404 and 23.337. Finding Critical Values Using StatCrunchClick on Stat > Calculators > Chi-Square Enter the degrees of freedom, the direction of the inequality, and the probability (leave X blank). Then press Compute. Example 2 Use the technology of your choice to find Χ20.01 with 20 degrees of freedom. [ reveal answer ] Χ20.01,20 ≈ 37.566 Constructing Confidence Intervals about σ2 and σNow that we have the basics of the distribution of the variable Χ2, we can work on constructing a formula for the confidence interval. From the distribution shape on the previous page, we know that of the Χ2 values will be between the two critical values shown below.This gives us the following inequality: If we solve the inequality for σ2, we get the formula for the confidence interval: A (1-α)100% confidence interval for σ2 is Note: The sample must be taken from a normally distributed population. Note #2: If a confidence interval for σ is desired, we can take the square root of each part. Now that we have the confidence interval formula, let's try a couple examples. Example 3 Suppose a sample of 30 ECC students are given an IQ test. If the sample has a standard deviation of 12.23 points, find a 90% confidence interval for the population standard deviation. Solution: We first need to find the critical values: andThen the confidence interval is: So we are 90% confident that the standard deviation of the IQ of ECC students is between 10.10 and 15.65 bpm. Finding Confidence Intervals Using StatCrunch
The confidence interval should be displayed. Note: If you need a confidence interval about the population standard deviation, take the square root of the values in the resulting confidence interval. Here's one for you to try: Example 4 In Example 3 in Section 9.1, we assumed the standard deviation of the resting heart rates of students was 10 bpm.
(Click here to view the data in a format more easily copied.) Use StatCrunch to find a 95% confidence interval for the standard deviation of the resting heart rates for students in this particular class. [ reveal answer ] Using StatCrunch, we get the following result: So the standard deviation is between and .What are the requirements for constructing a confidence interval?In order to find a confidence interval, the margin of error must be known. The margin of error depends on the degree of confidence that is required for the estimation. Typically degrees of confidence vary between 90% and 99.9%, but it is up to the researcher to decide.
What 3 conditions must be met before calculating a confidence interval?There are three conditions we need to satisfy before we make a one-sample z-interval to estimate a population proportion. We need to satisfy the random, normal, and independence conditions for these confidence intervals to be valid.
What are the 4 steps in constructing confidence intervals?How to Calculate Confidence Intervals. One-Sided Confidence Intervals vs. ... . Step #1: Find the number of samples (n). ... . Step #2: Calculate the mean (x) of the the samples. ... . Step #3: Calculate the standard deviation (s). ... . Step #4: Decide the confidence interval that will be used.. What are the requirements to construct a confidence interval for the population mean using the normal distribution?To get a 90% confidence interval, we must include the central 90% of the probability of the normal distribution. If we include the central 90%, we leave out a total of α = 10% in both tails, or 5% in each tail, of the normal distribution.
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