© 2002 Prentice-Hall, Inc.Chap 8-1 Statistics for Managers using Microsoft Excel 3 rd Edition Kafla 8 Próf fyrir tvö úrtök (Ekki þýtt)

© 2002 Prentice-Hall, Inc. Chap 8-2 Chapter Topics Comparing two independent samples Independent samples Z test for the difference in two means Pooled variance t test for the difference in two means F test for the difference in two variances Comparing two related samples Paired sample z test for the mean difference Paired sample t test for the mean difference

© 2002 Prentice-Hall, Inc. Chap 8-3 Chapter Topics Wilcoxon rank-sum test Difference in two medians (continued)

© 2002 Prentice-Hall, Inc. Chap 8-4 Comparing Two Independent Samples Different data sources Unrelated Independent Sample selected from one population has no effect or bearing on the sample selected from the other population Use the difference between 2 sample means Use Z test or pooled variance t test

© 2002 Prentice-Hall, Inc. Chap 8-5 Independent Sample Z Test (Variances Known) Assumptions Samples are randomly and independently drawn from normal distributions Population variances are known Test statistic

© 2002 Prentice-Hall, Inc. Chap 8-6 Independent Sample Z Test (Large Samples) Assumptions Samples are randomly and independently drawn Population variances either known or unknown Both sample sizes are at least 30 Test statistic

© 2002 Prentice-Hall, Inc. Chap 8-7 Independent Sample (Two Sample) Z Test in EXCEL Independent sample Z test with variances known Tools | data analysis | z-test: two sample for means Independent sample Z test with large sample Tools | data analysis | z-test: two sample for means If the population variances are unknown, use sample variances

© 2002 Prentice-Hall, Inc. Chap 8-8

© 2002 Prentice-Hall, Inc. Chap 8-9 Pooled Variance t Test (Variances Unknown) Assumptions Both populations are normally distributed Samples are randomly and independently drawn Population variances are unknown but assumed equal If both populations are not normal, need large sample sizes

© 2002 Prentice-Hall, Inc. Chap 8-10 Developing the Pooled Variance t Test Setting up the hypotheses H 0 :  1   2 H 1 :  1 >  2 H 0 :  1 -  2 = 0 H 1 :  1 -  2  0 H 0 :  1 =  2 H 1 :  1   2 H 0 :  1  2 H 0 :  1 -  2  0 H 1 :  1 -  2 > 0 H 0 :  1 -  2  H 1 :  1 -  2 < 0 OR Left Tail Right Tail Two Tail  H 1 :  1 <  2

© 2002 Prentice-Hall, Inc. Chap 8-11 Developing the Pooled Variance t Test Calculate the pooled sample variances as an estimate of the common population variance (continued)

© 2002 Prentice-Hall, Inc. Chap 8-12 Developing the Pooled Variance t Test Compute the sample statistic (continued) Hypothesized difference

© 2002 Prentice-Hall, Inc. Chap 8-13 Pooled Variance t Test: Example You’re a financial analyst for Charles Schwab. Is there a difference in dividend yield between stocks listed on the NYSE & NASDAQ? You collect the following data: NYSE NASDAQ Number Sample mean Sample std dev Assuming equal variances, is there a difference in average yield (  = 0.05)? © T/Maker Co.

© 2002 Prentice-Hall, Inc. Chap 8-14 Calculating the Test Statistic

© 2002 Prentice-Hall, Inc. Chap 8-15 Solution H 0 :  1 -  2 = 0 i.e. (  1 =  2 ) H 1 :  1 -  2  0 i.e. (  1   2 )  = 0.05 df = = 44 Critical Value(s): Test Statistic: Decision: Conclusion: Reject at  = 0.05 There is evidence of a difference in means. t Reject H

© 2002 Prentice-Hall, Inc. Chap 8-16 p -Value Solution p Value 2 (p Value is between.02 and.05)  (  = 0.05). Reject Z Reject  is between.01 and.025 Test Statistic 2.03 is in the Reject Region Reject =.025

© 2002 Prentice-Hall, Inc. Chap 8-17

© 2002 Prentice-Hall, Inc. Chap 8-18 Pooled Variance t Test in PHStat and Excel If the raw data is available Tools | data analysis | t-test: two sample assuming equal variances If only summary statistics are available PHStat | two-sample tests | t test for differences in two means...

© 2002 Prentice-Hall, Inc. Chap 8-19 Solution in EXCEL Excel workbook that performs the pooled variance t test

© 2002 Prentice-Hall, Inc. Chap 8-20 F Test for Difference in Two Population Variances Test for the difference in two independent populations Parametric test procedure Assumptions Both populations are normally distributed Test is not robust to this violation Samples are randomly and independently drawn

© 2002 Prentice-Hall, Inc. Chap 8-21 The F Test Statistic = Variance of Sample 1 n = degrees of freedom n = degrees of freedom F0 = Variance of Sample 2

© 2002 Prentice-Hall, Inc. Chap 8-22 Hypotheses H 0 :  1 2 =  2 2 H 1 :  1 2  2 2 Test Statistic F = S 1 2 /S 2 2 Two Sets of Degrees of Freedom df 1 = n 1 - 1; df 2 = n Critical Values: F L( ) and F U( ) F L = 1/F U * (*degrees of freedom switched) Developing the F Test Reject H 0  /2 Do Not Reject F 0 FLFL FUFU n 1 -1, n 2 -1

© 2002 Prentice-Hall, Inc. Chap 8-23 F Test: An Example You are a financial analyst for Charles Schwab. You want to compare dividend yields between stocks listed on the NYSE & NASDAQ. You collect the following data: NYSE NASDAQ Number 2125 Mean Std dev Is there a difference in the variances between the NYSE & NASDAQ at the  0.05 level? © T/Maker Co.

© 2002 Prentice-Hall, Inc. Chap 8-24 F Test: Example Solution Finding the critical values for  =.05

© 2002 Prentice-Hall, Inc. Chap 8-25 F Test: Example Solution H 0 :  1 2 =  2 2 H 1 :  1 2   2 2  .05 Df 1  20 df 2  24 Critical value(s): Test Statistic: Decision: Conclusion: Do not reject at  = 0.05 There is no evidence of a difference in variances. 0 F Reject

© 2002 Prentice-Hall, Inc. Chap 8-26 F Test in PHStat PHStat | two-sample tests | F test for differences in two variances Example in excel spreadsheet

© 2002 Prentice-Hall, Inc. Chap 8-27 F Test: One-Tail H 0 :  1 2   2 2 H 1 :  1 2  2 2 H 0 :  1 2   2 2 H 1 :  1 2 >  2 2 Reject .05 F 0 F 0 Reject .05  =.05 or Degrees of freedom switched

© 2002 Prentice-Hall, Inc. Chap 8-28 Comparing Two Related Samples Test the means of two related samples Paired or matched Repeated measures (before and after) Use difference between pairs Eliminates variation between subjects

© 2002 Prentice-Hall, Inc. Chap 8-29 Z Test for Mean Difference (Variance Known) Assumptions Both populations are normally distributed Observations are paired or matched Variance known Test statistic

© 2002 Prentice-Hall, Inc. Chap 8-30 t Test for Mean Difference (Variance Unknown) Assumptions Both populations are normally distributed Observations are matched or paired Variance unknown If population not normal, need large samples Test statistic

© 2002 Prentice-Hall, Inc. Chap 8-31 User Current Leader (1) New Software (2) Difference D i C.B.9.98 Seconds 9.88 Seconds.10 T.F M.H R.K M.O D.S S.S C.T K.T S.Z Paired Sample t Test: Example You work in the finance department. Is the new financial package faster (  =0.05 level)? You collect the following data entry times:

© 2002 Prentice-Hall, Inc. Chap 8-32 Paired Sample t Test: Example Solution Is the new financial package faster (0.05 level)?.084D = H 0 :  D   H 1 :  D     Test Statistic Critical Value= df = n - 1 = 9 Reject  Decision: Reject H 0 t Stat. in the rejection zone. Conclusion: The new software package is faster. 3.15

© 2002 Prentice-Hall, Inc. Chap 8-33 Paired Sample t Test in EXCEL Tools | data analysis… | t-test: paired two sample for means Example in excel spreadsheet