Introduction to SPSS. Katie Handwerger Why n How February 19, 2009

Transcription

1 Introduction to SPSS Katie Handwerger Why n How February 19, 2009

2 Overview Setting up a data file Frequencies/Descriptives One-sample T-test Paired-samples T-test Independent-samples T-test One-way ANOVA

3 Study Three Diagnostic Groups MDD, Bipolar, and Dysthymia Subjects receive either Drug or Placebo Measure of success: Beck Depression Inventory (BDI) score BDI Assessed at 3 points: Pre Treatment After 4 Weeks After 8 Weeks

4 Setting Up A Data File

5 Setting Up A Data File Important: Always Use Labels

6 Setting Variable Types

7 Setting Value Labels

8 Setting Value Labels Important: THESE VALUES ARE ARBITRARY!!

9 Accounting for Missing Variables Important: Account for Missing Variables with A Value That Would Never Be Found in the Dataset

10 Setting Up A Date File

11 Setting Up A Data File New Variable Labels Will Appear Up Top

12 Setting Up A Data File

13 Viewing Value Labels

14 Viewing Value Labels

15 Now we have our data set up What can we do with it???

16 G 1 g 2 g 3 g 4 g 5 g 6 g 7 g 8 g 9 g 1 0 g 1 2 g 1 3 g 1 4 g 1 5 G 1 g 2 g 3 g 4 g 5 g 6 g 7 g 8 g 9 g 1 0 g 1 2 g 1 3 g 1 4 g 1 5 Descriptives Mean Median Mode Range Skewness Measure of the asymmetry of the probability distribution Good way to check for outliers 16 Negative Skew Positive Skew G 1 g 2 g 3 g 4 g 5 g 6 g 7 g 8 g 9 g 1 0 g 1 2 g 1 3 g 1 4 g 1 5 0

17 Running Descriptives

18 Running Descriptives

19 Running Descriptives

20 Descriptives Output

21 Descriptives Output

22 Descriptives Output Missing Values Accounted For

23 Selecting Only Certain Cases

24 Selecting Only Certain Cases What if I only want to include a certain population in my analyses? i.e. people with SEVERE Depression Defined as: BDI>40 at pre treatment

25 Selecting Only Certain Cases

26 Selecting Only Certain Cases

27 Selecting Only Certain Cases

28 Selecting All Cases IMPORTANT NOTE: All analyses after you select certain cases will only include those cases UNTIL you go back and select ALL

29 Selecting All Cases

30 Computing New Variables I have BDI Scores at Pre, Week 4, and Week 8 What if I want to have a new column that gives me the change from: Pre to Week 4? Pre to Week 8?

31 Computing New Variables

32 Computing New Variables

33 Computing New Variables

34 Computing New Variables Don t Forget To Give Your New Variable a Label!

35 Computing New Variables

36 Computing New Variables

37 Computing New Variables

38 T-Tests Paired-Samples Assesses the difference between the two variables for each case Tests to see if the average difference is sig different from zero. One Sample Comparing Mean Scores to an existing predetermined unit Independent-Samples Comparing Mean of Two Groups (IV) on a DV

39 Paired T-Tests Two continuous variables i.e. In the Tx group alone, did subjects display a sig. change in BDI score from pre to Week 4? Week 8? Is the change (difference) from Time 1 to Time 2 is sig. different from zero

40 Selecting Cases

41 Paired-Samples T-Test

42 Paired-Samples T-Test

43 Paired-Samples T-Test

44 One-Sample T-Test Question: Does our sample as a whole at pre have BDI scores significantly different from what clinicians deem to be the cutoff for severe depression (BDI=30)?

45 One-Sample T-Test

46 One-Sample T-Test

47 One-Sample T-Test

48 Independent Samples T-Test Do two groups different on a continuous measure? IV = Categorical (Only 2 Groups) DV = Continuous i.e. Does our sample differ in any important measure pre-treatment on the basis of sex? i.e. BDI Scores Pre-Treatment IV = Sex DV = Pre BDI Score

49 Independent-Sample T-Test

50 Independent-Sample T-Test

51 Independent-Sample T-Test

52 Independent Samples T-Test Is there a significant difference in the BDI change score from Pre to Week 4 between those who received the drug and those who were in the placebo group? IV = Treatment Group (drug vs placebo) DV = BDI Change Score

53 Independent-Sample T-Test

54 Independent-Sample T-Test

55 Levene s Test: homogeneity of variance

56 Levene s Test: homogeneity of variance

57 One-Way ANOVA IV = Categorical DV = Continuous Two or more groups Note: An Ind-T-Test can only have 2 groups i.e. Did the three Diagnostic Groups Differ in Pre-Treatment BDI Scores?

58 One-Way ANOVA

59 One-Way ANOVA

60 One-Way ANOVA

61 One-Way ANOVA

62 One-Way ANOVA

63 Post Hoc Tests

64 One-Way ANOVA

65 One-Way ANOVA NOTE: Don t Run Post-Hoc If p=ns

66 Comparing Groups So we know that the change from Pre to Week 8 was not sig. different between the groups BUT, was each group s change statistically significant?

67 Comparing Groups

68 Comparing Groups

69 Comparing Groups

70 Comparing Groups

71 Comparing Groups

72 Correlations Strength and direction of a linear relationship between two continuous variables Pearson product-moment correlation dividing the covariance of the two variables by the product of their SD

73 Correlations

74 Correlations

75 Correlations

76 Do NOT Run Correlations on Categorical Variables!! Important: THESE VALUES ARE ARBITRARY!!

77 Correlations What NOT To Do

78 Correlations What NOT To Do

79 The End