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Statistics for Business and Economics Chapter 9 Categorical Data Analysis

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Learning Objectives 1.Explain 2 Test for Proportions 2.Explain 2 Test of Independence 3.Solve Hypothesis Testing Problems More Than Two Population Proportions Independence

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Data Types Data QuantitativeQualitative ContinuousDiscrete

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Qualitative Data Qualitative random variables yield responses that classify –Example: gender (male, female) Measurement reflects number in category Nominal or ordinal scale Examples –What make of car do you drive? –Do you live on-campus or off-campus?

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Hypothesis Tests Qualitative Data Qualitative Data Z Test c 2 Test ProportionIndependence 1 pop. c 2 Test More than 2 pop.

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Chi-Square ( 2 ) Test for k Proportions

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Hypothesis Tests Qualitative Data Qualitative Data Z Test c 2 Test ProportionIndependence 1 pop. c 2 Test More than 2 pop.

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Multinomial Experiment n identical trials k outcomes to each trial Constant outcome probability, p k Independent trials Random variable is count, n k Example: ask 100 people (n) which of 3 candidates (k) they will vote for

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Chi-Square ( 2 ) Test for k Proportions Tests equality (=) of proportions only –Example: p 1 =.2, p 2 =.3, p 3 =.5 One variable with several levels Uses one-way contingency table

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One-Way Contingency Table Shows number of observations in k independent groups (outcomes or variable levels) Outcomes (k = 3) Number of responses Candidate TomBillMaryTotal

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Conditions Required for a Valid Test: One-way Table 1.A multinomial experiment has been conducted 2.The sample size n is large: E(n i ) is greater than or equal to 5 for every cell

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2 Test for k Proportions Hypotheses & Statistic 2.Test Statistic Observed count Expected count: E(n i ) = np i,0 3.Degrees of Freedom: k – 1 Number of outcomes Hypothesized probability 1.Hypotheses H 0 : p 1 = p 1,0, p 2 = p 2,0,..., p k = p k,0 H a : At least one p i is different from above

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2 Test Basic Idea 1.Compares observed count to expected count assuming null hypothesis is true 2.Closer observed count is to expected count, the more likely the H 0 is true Measured by squared difference relative to expected count Reject large values

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Finding Critical Value Example What is the critical 2 value if k = 3, and =.05? c 2 0 Upper Tail Area DF.995….95… …0.004… …0.103… Table (Portion) If n i = E(n i ), 2 = 0. Do not reject H 0 df= k - 1 = Reject H 0 =.05

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As personnel director, you want to test the perception of fairness of three methods of performance evaluation. Of 180 employees, 63 rated Method 1 as fair, 45 rated Method 2 as fair, 72 rated Method 3 as fair. At the.05 level of significance, is there a difference in perceptions? 2 Test for k Proportions Example

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H 0 : H a : = n 1 = n 2 = n 3 = Critical Value(s): Test Statistic: Decision: Conclusion: p 1 = p 2 = p 3 = 1/3 At least 1 is different =.05 c 2 0 Reject H Test for k Proportions Solution

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Test Statistic: Decision: Conclusion: 2 = 6.3 Reject at =.05 There is evidence of a difference in proportions 2 Test for k Proportions Solution H 0 : H a : = n 1 = n 2 = n 3 = Critical Value(s): c 2 0 Reject H 0 p 1 = p 2 = p 3 = 1/3 At least 1 is different =.05

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Contingency Tables Useful in situations involving multiple population proportions Used to classify sample observations according to two or more characteristics Also called a cross-classification table.

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Contingency Table Example Left-Handed vs. Gender Dominant Hand: Left vs. Right Gender: Male vs. Female 2 categories for each variable, so called a 2 x 2 table Suppose we examine a sample of 300 children

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Contingency Table Example Sample results organized in a contingency table: (continued) Gender Hand Preference LeftRight Female Male Females, 12 were left handed 180 Males, 24 were left handed sample size = n = 300:

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2 Test for the Difference Between Two Proportions If H 0 is true, then the proportion of left-handed females should be the same as the proportion of left-handed males The two proportions above should be the same as the proportion of left-handed people overall H 0 : π 1 = π 2 ( Proportion of females who are left handed is equal to the proportion of males who are left handed ) H 1 : π 1 π 2 ( The two proportions are not the same hand preference is not independent of gender )

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The Chi-Square Test Statistic where: f o = observed frequency in a particular cell f e = expected frequency in a particular cell if H 0 is true (Assumed: each cell in the contingency table has expected frequency of at least 5) The Chi-square test statistic is:

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Decision Rule 2 α Decision Rule: If, reject H 0, otherwise, do not reject H 0 The test statistic approximately follows a chi- squared distribution with one degree of freedom 0 Reject H 0 Do not reject H 0

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Computing the Average Proportion Here: 120 Females, 12 were left handed 180 Males, 24 were left handed i.e., of all the children the proportion of left handers is 0.12, that is, 12% The average proportion is:

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Finding Expected Frequencies To obtain the expected frequency for left handed females, multiply the average proportion left handed (p) by the total number of females To obtain the expected frequency for left handed males, multiply the average proportion left handed (p) by the total number of males If the two proportions are equal, then P(Left Handed | Female) = P(Left Handed | Male) =.12 i.e., we would expect (.12)(120) = 14.4 females to be left handed (.12)(180) = 21.6 males to be left handed

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Observed vs. Expected Frequencies Gender Hand Preference LeftRight Female Observed = 12 Expected = 14.4 Observed = 108 Expected = Male Observed = 24 Expected = 21.6 Observed = 156 Expected =

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Gender Hand Preference LeftRight Female Observed = 12 Expected = 14.4 Observed = 108 Expected = Male Observed = 24 Expected = 21.6 Observed = 156 Expected = The Chi-Square Test Statistic The test statistic is:

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Decision Rule Decision Rule: If > 3.841, reject H 0, otherwise, do not reject H 0 Here, = < = 3.841, so we do not reject H 0 and conclude that there is not sufficient evidence that the two proportions are different at = = Reject H 0 Do not reject H 0

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Extend the 2 test to the case with more than two independent populations: 2 Test for Differences Among More Than Two Proportions H 0 : π 1 = π 2 = … = π c H 1 : Not all of the π j are equal (j = 1, 2, …, c)

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The Chi-Square Test Statistic Where: f o = observed frequency in a particular cell of the 2 x c table f e = expected frequency in a particular cell if H 0 is true (Assumed: each cell in the contingency table has expected frequency of at least 1) The Chi-square test statistic is:

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Computing the Overall Proportion The overall proportion is: Expected cell frequencies for the c categories are calculated as in the 2 x 2 case, and the decision rule is the same: Where is from the chi- squared distribution with c – 1 degrees of freedom Decision Rule: If, reject H 0, otherwise, do not reject H 0

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The Marascuilo Procedure Used when the null hypothesis of equal proportions is rejected Enables you to make comparisons between all pairs Start with the observed differences, p j – p j, for all pairs (for j j )......then compare the absolute difference to a calculated critical range

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2 Test of Independence

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Hypothesis Tests Qualitative Data Qualitative Data Z Test c 2 Test ProportionIndependence 1 pop. c 2 Test More than 2 pop.

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2 Test of Independence Shows if a relationship exists between two qualitative variables –One sample is drawn –Does not show causality Uses two-way contingency table

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2 Test of Independence Contingency Table Shows number of observations from 1 sample jointly in 2 qualitative variables Levels of variable 2 Levels of variable 1

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Conditions Required for a Valid 2 Test: Independence 1.Multinomial experiment has been conducted 2.The sample size, n, is large: E ij is greater than or equal to 5 for every cell

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2 Test of Independence Hypotheses & Statistic 1.Hypotheses H 0 : Variables are independent H a : Variables are related (dependent) 3.Degrees of Freedom: (r – 1)(c – 1) Rows Columns 2.Test Statistic Observed count Expected count

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2 Test of Independence Expected Counts 1.Statistical independence means joint probability equals product of marginal probabilities 2.Compute marginal probabilities and multiply for joint probability 3.Expected count is sample size times joint probability

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Marginal probability = Expected Count Example Location UrbanRural House Style Obs. Obs.Total Split–Level Ranch Total

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Marginal probability = Expected Count Example Marginal probability = Location UrbanRural House Style Obs. Obs.Total Split–Level Ranch Total

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Expected Count Example Marginal probability = Marginal probability = Joint probability = Location UrbanRural House Style Obs. Obs.Total Split–Level Ranch Total Expected count = 160· = 54.6

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Expected Count Calculation House Location Urban Rural House Style Obs. Exp. Obs. Exp. Total Split-Level · · Ranch 15 48· · Total

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As a realtor you want to determine if house style and house location are related. At the.05 level of significance, is there evidence of a relationship? 2 Test of Independence Example

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2 Test of Independence Solution H 0 : H a : = df = Critical Value(s): Test Statistic: Decision: Conclusion: No Relationship Relationship.05 (2 - 1)(2 - 1) = 1 c 2 0 Reject H =.05

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E ij 5 in all cells 2 Test of Independence Solution 112· · · ·78 160

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2 Test of Independence Solution

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Test Statistic: Decision: Conclusion: 2 = 8.41 Reject at =.05 There is evidence of a relationship H 0 : H a : = df = Critical Value(s): c 2 0 Reject H 0 No Relationship Relationship.05 (2 - 1)(2 - 1) = =.05

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Youre a marketing research analyst. You ask a random sample of 286 consumers if they purchase Diet Pepsi or Diet Coke. At the.05 level of significance, is there evidence of a relationship? 2 Test of Independence Thinking Challenge Diet Pepsi Diet CokeNoYesTotal No Yes Total

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2 Test of Independence Solution* H 0 : H a : = df = Critical Value(s): Test Statistic: Decision: Conclusion: No Relationship Relationship.05 (2 - 1)(2 - 1) = 1 c 2 0 Reject H =.05

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E ij 5 in all cells 170· · · · Test of Independence Solution*

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Test Statistic: Decision: Conclusion: 2 = Reject at =.05 There is evidence of a relationship H 0 : H a : = df = Critical Value(s): c 2 0 Reject H 0 No Relationship Relationship.05 (2 - 1)(2 - 1) = =.05

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There is a statistically significant relationship between purchasing Diet Coke and Diet Pepsi. So what do you think the relationship is? Arent they competitors? 2 Test of Independence Thinking Challenge 2 Diet Pepsi Diet CokeNoYesTotal No Yes Total

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Low Income You Re-Analyze the Data High Income Diet Pepsi Diet Coke No Yes Total No Yes Total Diet Pepsi Diet Coke No Yes Total No Yes Total

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True Relationships* Apparent relation Underlying causal relation Control or intervening variable (true cause) Diet Coke Diet Pepsi

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Moral of the Story* © T/Maker Co. Numbers dont think - People do!

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Conclusion 1.Explained 2 Test for Proportions 2.Explained 2 Test of Independence 3.Solved Hypothesis Testing Problems More Than Two Population Proportions Independence

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