Probability and Statistics Notes

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1 Probability and Statistics Notes Chapter Seven Jesse Crawford Department of Mathematics Tarleton State University Spring 2011 (Tarleton State University) Chapter Seven Notes Spring / 42

2 Outline 1 General Hypothesis Testing Concepts 2 Section 7.2: Tests about One Mean 3 More General Concepts: Test Statistics and p-values 4 Section 7.1: Tests about Proportions 5 Section 7.3: Tests of the Equality of Two Means 6 Section 7.4: Tests for Variances 7 Sections 6.7, 6.8, and 7.7: Linear Regression (Tarleton State University) Chapter Seven Notes Spring / 42

3 Statistical testing problems usually involve two hypotheses about a parameter, such as H 0 : µ 150 vs. H 1 : µ > 150. Here, the parameter µ represents the average mass of an apple produced using a new type of fertilizer. Assuming that 150 grams is the mass of an apple produced using the old fertilizer, H 0 represents a hypothesis of no change, and H 0 is called the null hypothesis. H 1 is the hypothesis that the fertilizer manufacturer would need to prove to convince people to buy this type of fertilizer. H 1 is called the alternative hypothesis. (Tarleton State University) Chapter Seven Notes Spring / 42

4 For mathematical reasons, the testing problem is usually restated as H 0 : µ = 150 vs. H 1 : µ > 150. (Replaced with = in H 0 ) Two types of errors: Type I: Reject H0 when H 0 is true. Type II: Don t reject H0 when H 0 is false. (We never accept H 0 ) We can t decrease the chance of both types of errors without raising the sample size. (Tarleton State University) Chapter Seven Notes Spring / 42

5 Solution: settle for making P[Type I error] small. We choose a number α, called the significance level of the test, and construct the test so that P[Type I error] = α. A conventional value of α is 0.05, but other values may be more appropriate, depending on the situation. If α is small, rejecting H 0 is strong evidence that H 0 is false, and the smaller α is, the stronger the evidence is. Unfortunately, not rejecting H 0 is not strong evidence that H 0 is true. This is a more delicate matter that s addressed in chapter 10. Hypothesis tests with significance level α correspond to confidence intervals with confidence level 1 α. (Tarleton State University) Chapter Seven Notes Spring / 42

6 The remaining slides in this section are not essential to material in chapter 7. P[Type II error] = β. Among all tests with significance level α, we would like the one that minimizes β. K = 1 β is called the power of the test. Minimizing β is equivalent to maximizing power, so we want a most powerful test. Notice that β is actually a function of the unknown parameter. (Tarleton State University) Chapter Seven Notes Spring / 42

7 For example, let s return to H 0 : µ = 150 vs. H 1 : µ > 150, and let s assume the population standard deviation is σ = 10. If µ is actually equal to , then H 0 is false, but the chance of rejecting H 0 will be low, unless the sample size is extremely large. The chance of making a type II error would be large and the power would be low. If µ is actually equal to 300, rejecting H 0 would be very likely, even with a small sample size. The chance of making a type II error would be small and the power would be high. This shows how β and the power depend on the unknown parameter. The tests studied in this chapter maximize power for each value of the unknown parameter, so they are called uniformly most powerful tests, a topic discussed in more detail in chapter 10. (Tarleton State University) Chapter Seven Notes Spring / 42

8 The power of these tests is still low if the true parameter is close to H 0 (relative to the population variance and the sample size). In these cases, achieving a high power level is not possible. (Maximum power does not mean high power. If a high power is impossible, the maximum will still be low.) In other words, even if we don t reject H 0, it is always possible that H 0 is false, but the true parameter is so close to H 0 that it appears that H 0 is true. For this reason we can never accept H 0. (Tarleton State University) Chapter Seven Notes Spring / 42

9 Outline 1 General Hypothesis Testing Concepts 2 Section 7.2: Tests about One Mean 3 More General Concepts: Test Statistics and p-values 4 Section 7.1: Tests about Proportions 5 Section 7.3: Tests of the Equality of Two Means 6 Section 7.4: Tests for Variances 7 Sections 6.7, 6.8, and 7.7: Linear Regression (Tarleton State University) Chapter Seven Notes Spring / 42

10 Let µ be the mean of a population, and consider the hypothesis test H 0 : µ = µ 0 vs. H 1 : µ µ 0. Suppose X 1,..., X n is a random sample from this population, and assume that one of the following conditions holds: n < 30 and the population is normal, or n 30. If σ is unknown, the decision rule is where T is the test statistic This is a two-tailed test. Reject H 0 if T t α/2 (n 1), T = X µ 0 S/ n. (Tarleton State University) Chapter Seven Notes Spring / 42

11 For the testing problem reject H 0 if T > t α (n 1). For the testing problem reject H 0 if T < t α (n 1). These are one-tailed tests. H 0 : µ = µ 0 vs. H 1 : µ > µ 0, H 0 : µ = µ 0 vs. H 1 : µ < µ 0, If σ is known, replace s with σ and t α (n 1) with z α. Recall that if n is large, t α (n 1) can also be replaced by z α as an approximation. (Tarleton State University) Chapter Seven Notes Spring / 42

12 Outline 1 General Hypothesis Testing Concepts 2 Section 7.2: Tests about One Mean 3 More General Concepts: Test Statistics and p-values 4 Section 7.1: Tests about Proportions 5 Section 7.3: Tests of the Equality of Two Means 6 Section 7.4: Tests for Variances 7 Sections 6.7, 6.8, and 7.7: Linear Regression (Tarleton State University) Chapter Seven Notes Spring / 42

13 Test Statistics Recall that a parameter is a number (or vector etc.) related to the population. For example, the population mean µ and population variance σ 2 are parameters. Parameters are not random. A statistic is a number (or vector etc.) whose value is based on a random sample. For example, the sample mean X and sample variance S 2 are statistics. Statistics are random variables. Most decision rules for hypothesis tests are based on test statistics. For example, the decision rules from the previous section were based on the test statistic T = X µ 0 s/ n. (Tarleton State University) Chapter Seven Notes Spring / 42

14 p-values Once a sample has been collected and a test statistic has been calculated, the p-value of the test can also be calculated. Definition The p-value is the probability of obtaining a test statistic at least as extreme as the one that was actually observed, assuming the null hypothesis is true. (Tarleton State University) Chapter Seven Notes Spring / 42

15 Outline 1 General Hypothesis Testing Concepts 2 Section 7.2: Tests about One Mean 3 More General Concepts: Test Statistics and p-values 4 Section 7.1: Tests about Proportions 5 Section 7.3: Tests of the Equality of Two Means 6 Section 7.4: Tests for Variances 7 Sections 6.7, 6.8, and 7.7: Linear Regression (Tarleton State University) Chapter Seven Notes Spring / 42

16 Let p be a population proportion, and consider the testing problem H 0 : p = p 0 vs. H : p p 0. Suppose ˆp is the sample proportion from a sample of size n, and assume that both nˆp 5 and n(1 ˆp) 5. Decision rule: where Z is the test statistic Reject H 0 if Z z α/2, Z = ˆp p 0 p0 (1 p 0 )/n. Use appropriate modifications for one-tailed tests. (Tarleton State University) Chapter Seven Notes Spring / 42

17 Let p 1 and p 2 be two population proportions, and consider H 0 : p 1 = p 2 vs. H 1 : p 1 p 2. Let ˆp 1 = Y 1 /n 1 and ˆp 2 = Y 2 /n 2 be corresponding sample proportions based on independent samples of sizes n 1 and n 2, respectively. Also, assume that both n i ˆp i 5 and n i (1 ˆp i ) 5, for i = 1, 2. Decision rule: Reject H 0 if Z z α/2, where Z = ˆp 1 ˆp 2 ( ), and ˆp(1 ˆp) n1 n2 ˆp = Y 1 + Y 2 n 1 + n 2. (Tarleton State University) Chapter Seven Notes Spring / 42

18 Outline 1 General Hypothesis Testing Concepts 2 Section 7.2: Tests about One Mean 3 More General Concepts: Test Statistics and p-values 4 Section 7.1: Tests about Proportions 5 Section 7.3: Tests of the Equality of Two Means 6 Section 7.4: Tests for Variances 7 Sections 6.7, 6.8, and 7.7: Linear Regression (Tarleton State University) Chapter Seven Notes Spring / 42

19 Let N(µ X, σx 2 ) and N(µ Y, σy 2 ) be two normal populations, with the same, unknown variance σ = σ X = σ Y, and consider H 0 : µ X = µ Y vs. H 1 : µ X µ Y. Let X 1,..., X n and Y 1,..., Y m be independent samples from these populations. Decision rule: Reject H 0 if T t α/2 (n + m 2), where S p = T = X Y, and 1 S p n + 1 m (n 1)SX 2 + (m 1)S2 Y. n + m 2 This test is valid for any sample sizes, but it s only necessary to use this test for small sample sizes. (Tarleton State University) Chapter Seven Notes Spring / 42

20 Let N(µ X, σx 2 ) and N(µ Y, σy 2 ) be two normal populations, with known variances σ X and σ Y, and consider H 0 : µ X = µ Y vs. H 1 : µ X µ Y. Let X 1,..., X n and Y 1,..., Y m be independent samples from these populations. Decision rule: Reject H 0 if Z z α/2, where Z = X Y σ 2 Xn + σ2 Ym. This test is valid for any sample sizes. (Tarleton State University) Chapter Seven Notes Spring / 42

21 Consider two populations with means µ X and µ Y and variances σ X and σ Y. Consider H 0 : µ X = µ Y vs. H 1 : µ X µ Y. Let X 1,..., X n and Y 1,..., Y m be independent samples from these populations, where n 30 and m 30. Decision rule: Reject H 0 if Z z α/2, where Z = X Y. + S2 Y m S 2 X n This test is only valid for large sample sizes. If the population variances are known, use them in Z instead of the sample variances. (Tarleton State University) Chapter Seven Notes Spring / 42

22 Consider a sample of paired observations (X 1, Y 1 ),..., (X n, Y n ) and H 0 : µ X = µ Y vs. H 1 : µ X µ Y. Define the differences D i = X i Y i, for i = 1,..., n. The above testing problem is now equivalent to testing H 0 : µ D = 0 vs. H 1 : µ D 0, and can be solved using the techniques from Section 7.2. Decision rule: Reject H 0 if T t α/2 (n 1), where T = D S D / n. This test is only valid if the populations are normal or n 30. (Tarleton State University) Chapter Seven Notes Spring / 42

23 Outline 1 General Hypothesis Testing Concepts 2 Section 7.2: Tests about One Mean 3 More General Concepts: Test Statistics and p-values 4 Section 7.1: Tests about Proportions 5 Section 7.3: Tests of the Equality of Two Means 6 Section 7.4: Tests for Variances 7 Sections 6.7, 6.8, and 7.7: Linear Regression (Tarleton State University) Chapter Seven Notes Spring / 42

24 Let N(µ X, σx 2 ) be a normal population, and consider H 0 : σ 2 X = σ2 0 vs. H 1 : σ 2 X σ2 0. Let X 1,..., X n be a random sample from this population. Decision rule: Reject H 0 if χ 2 χ 2 1 α/2 (n 1) or χ2 χ 2 α/2 (n 1), where χ 2 = (n 1)S2 σ0 2. This test is valid for any sample size. This test is not robust against deviations from normality. (Tarleton State University) Chapter Seven Notes Spring / 42

25 Let N(µ X, σx 2 ) and N(µ Y, σy 2 ) be two normal populations, and consider H 0 : σx 2 = σ2 Y vs. H 1 : σx 2 σ2 Y. Let X 1,..., X n and Y 1,..., Y m be independent samples from these populations. Decision rule: Reject H 0 if F F 1 α/2 (n 1, m 1) or F F α/2 (n 1, m 1), where F = S2 X SY 2. This test is valid for any sample sizes. This test is not robust against deviations from normality. (Tarleton State University) Chapter Seven Notes Spring / 42

26 Outline 1 General Hypothesis Testing Concepts 2 Section 7.2: Tests about One Mean 3 More General Concepts: Test Statistics and p-values 4 Section 7.1: Tests about Proportions 5 Section 7.3: Tests of the Equality of Two Means 6 Section 7.4: Tests for Variances 7 Sections 6.7, 6.8, and 7.7: Linear Regression (Tarleton State University) Chapter Seven Notes Spring / 42

27 Simple Linear Regression A simple linear regression model is given by Y i = a + bx i + ɛ i, for i = 1,..., n, where Y 1,..., Y n and x 1,..., x n are measurements of some physical quantities. We are interested in finding a relationship between these quantities and using x to predict Y. The Y i s are random variables, while the x i s can be regarded as random or constant. Both the Y i s and x i s are observable. x is often called the independent, explanatory, or input variable. Y is often called the dependent, response, or output variable. (Tarleton State University) Chapter Seven Notes Spring / 42

28 Simple Linear Regression Y i = a + bx i + ɛ i, for i = 1,..., n, where This model assumes there is a linear relationship between x and Y, except for random error. The parameters a and b are the y-intercept and slope of this line. a and b are fixed numbers. a and b are not observable. (Tarleton State University) Chapter Seven Notes Spring / 42

29 Simple Linear Regression Y i = a + bx i + ɛ i, for i = 1,..., n, where The ɛ i s are random errors. We will assume that ɛ 1,..., ɛ n are IID N(0, σ 2 ) random variables. σ 2 is a parameter (it s just a number, not random). The ɛ i s and σ 2 are not observable. (Tarleton State University) Chapter Seven Notes Spring / 42

30 Simple Linear Regression: Concise Formulation A simple linear regression model is given by Y i = a + bx i + ɛ i, for i = 1,..., n, where Y 1,..., Y n are observable random variables; x 1,..., x n are observable numbers; a and b are unobservable numbers; ɛ 1,..., ɛ n are unobservable IID N(0, σ 2 ) random variables; and σ 2 is an unobservable positive number. (Tarleton State University) Chapter Seven Notes Spring / 42

31 Hook s Law Hook s Law from physics states that the length of a spring is a linear function of the mass placed on the spring. Consider the following data. Mass (kg) Length (cm) Does the data fall exactly on a line? Why not? Find the slope and y-intercept of the best fitting line. (Tarleton State University) Chapter Seven Notes Spring / 42

32 Estimating the y-intercept and Slope For the simple linear regression model, the MLEs for a and b are least squares estimates: they minimize the sum of the squares of the distances from the data points to the regression line. ˆb = r S Y xi y i 1/n( x i )( y i ) = S x x 2 i 1/n(. x i ) 2 The regression line passes through the vector of means (x, Y ). Therefore, â = Y ˆbx. Note that â and ˆb are observable random variables. (Tarleton State University) Chapter Seven Notes Spring / 42

33 Estimates for Hook s Law Example For the Hook s Law example, ˆb = and â = Scatter plot and least squares line: How would we estimate σ 2? (Tarleton State University) Chapter Seven Notes Spring / 42

34 Residuals and Estimating σ 2 Rearranging the regression equation, we obtain ɛ i = Y i (a + bx i ), for i = 1,..., n. We can t directly observe the ɛ i s. We can observe the residuals, given by the equation below: e i = Y i (â + ˆbx i ), for i = 1,..., n. The residuals are observable random variables. If our estimates are accurate, e i ɛ i. The (unbiased) MLE for σ 2 is ˆσ 2 = 1 n 2 n ei 2. i=1 (Tarleton State University) Chapter Seven Notes Spring / 42

35 Strong Positive Correlation (Tarleton State University) Chapter Seven Notes Spring / 42

36 Strong Negative Correlation (Tarleton State University) Chapter Seven Notes Spring / 42

37 Virtually No Correlation (Tarleton State University) Chapter Seven Notes Spring / 42

38 Multiple Regression Y i = β 1 x i1 + β 2 x i2 + + β p x ip + ɛ i, for i = 1,..., n, where ɛ 1,..., ɛ n are unobservable IID N(0, σ 2 ) random variables. Y 1. Y n = X is called the design matrix. X 11 X 1p.. X n1 X np Y = Xβ + ɛ β 1. β p ɛ 1 +. ɛ n (Tarleton State University) Chapter Seven Notes Spring / 42

39 Y = Xβ + ɛ ˆβ = (X t X) 1 X t Y cov( ˆβ) = σ 2 (X t X) 1 The standard error of ˆβ j is the square root of the jth diagonal entry of cov( ˆβ). SE( ˆ ˆβ j ) = ˆσ [(X t X) 1 ] jj The following random variable has a t-distribution with n p degrees of freedom. T = ˆβ j β j SE( ˆ ˆβ j ). (Tarleton State University) Chapter Seven Notes Spring / 42

40 The following random variable has a t-distribution with n p degrees of freedom. 1 α confidence interval of β j : T = ˆβ j β j SE( ˆ ˆβ j ). ˆβ j ± t α/2 (n p) ˆ SE( ˆβ j ) To test H 0 : β j = 0 vs. H 1 : β j 0, compute and reject if T t α/2 (n p). ˆβ j T = SE( ˆ ˆβ j ), (Tarleton State University) Chapter Seven Notes Spring / 42

41 The F -test Consider the multiple regression model Y i = β 1 x i1 + β 2 x i2 + + β p x ip + ɛ i, for i = 1,..., n. Let p 0 be a positive integer such that 1 p 0 p. Consider testing the null hypothesis that the first p 0 components in the regression equation are zero, i.e., H 0 : β 1 = = β p0 = 0 vs. H 1 : β j 0, for at least one j = 1,..., p 0. Let e be the vector of residuals for the full model. Let e (s) be the vector of residuals for the small model. The F-statistic used for this testing problem is F = ( e(s) 2 e 2 )/p 0 e 2. /(n p) (Tarleton State University) Chapter Seven Notes Spring / 42

42 The F -test H 0 : β 1 = = β p0 = 0 vs. H 1 : β j 0, for at least one j = 1,..., p 0. The F-statistic used for this testing problem is F = ( e(s) 2 e 2 )/p 0 e 2. /(n p) Under the null hypothesis, F has an F(p 0, n p) distribution. (Tarleton State University) Chapter Seven Notes Spring / 42

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