Frequently Asked Questions

1. [:FAQ/distFAQ:Questions about distributions of random variables]
2. [:FAQ/anovaFAQ:Questions involving analysis of variance]
3. [:FAQ/graphFAQ:Questions about graphics]
4. [:FAQ/corrsFAQ:Questions about correlations]
5. [:FAQ/regFAQ:Questions about regression]
6. [:FAQ/cdaFAQ:Questions about categorical data analysis]
7. [:FAQ/powerFAQ:Questions about p-values, significance, or power of tests]
8. [:FAQ/nonpFAQ:Questions about nonparametric tests]
9. [:FAQ/mvFAQ:Questions about multivariate analyses]

The FAQs below will be divided into categories including those above.

1. [:FAQ/Sattherthwaite:Why are the degrees of freedom not whole numbers?]

2. [:FAQ/stars:Why does SPSS give a row of asterisks instead of a mean in the outputted pivot table?]
3. [:FAQ/E:What does a number containing an 'E' signify?]

4. [:FAQ/LogitNPlusHalf:How do I explain adding 0.5 to the cells for the d' or logit transform?]

5. [:FAQ/heterogeneity: What is heterogeneity of variance in SPSS Probit and Logit procedures?]
6. [:FAQ/MatchedPairs: How do I do a matched pairs comparison on dichotomous data using covariates in SPSS?]

7. [:FAQ/SumsOfSquaresTypes:In ANOVA and Regression, what do the various different types of Sums of Squares mean, and does the choice matter?]

8. [:FAQ/Rss:Sums of squares used by R in lm, lmer and aov]

9. [:FAQ/poolse:How do I obtain a pooled standard error of the mean?]
10. [:FAQ/FVars: What is the variance of the mean of my transformed data?]
11. [:FAQ/rmform: How do I format data for input into a repeated measures analysis in SPSS?]
12. [:FAQ/glmrm: How do I perform a repeated measures analysis of variance in SPSS?]

13. [:FAQ/Interaction:How do I work out the meaning of a significant interaction?]

14. [:FAQ/SpssMancova:Aspects of ANCOVA (Analysis of Covariance)]

15. [:FAQ/ancreg:Can I do an analysis of covariance using a regression?]

16. [:FAQ/Manova:How do I adjust for varying covariates in a repeated measures ANOVA in SPSS?]

17. [:FAQ/Auc: What summary measures can I use to describe repeated measures?]
18. [:FAQ/intercept: Interpreting the intercept term in a regression when covarying out predictors of within-subject difference scores]
19. [:FAQ/beta: How do I compute linear trend coefficients for single cases?]
20. [:FAQ/singcase: How do I compare a within subjects group difference to that of a single case?]
21. [:FAQ/Tukey: How do I interpret subsets in Tukey's HSD output?]

22. [:FAQ/SimpleMainEffects:Why won't SPSS let me do post hoc tests involving a within-subjects factor?]

23. [:FAQ/SimpleEffs:How do I do a simple effects analysis in SPSS?]

24. [:FAQ/SobelTest:How do I test whether one independent variable has an influence on a dependent variable other than via the mediation of a second independent variable? (Sobel Test)]

25. [:FAQ/ChiSquaredSpss:How do I get SPSS to do a Chi-squared analysis of a two-way frequency table?]

26. [:FAQ/AdjRes:How do I know which elements contribute to a relationship in a two-way frequency table?]

27. [:FAQ/ChiExact: When do I use Fisher's exact test instead of chi-square?]
28. [:FAQ/chiwww: How do I obtain Fisher's exact test and chi-square for a two-way table in EXCEL?]

29. [:FAQ/ChiTrend:How do I do a linear trend of proportions in a Chi-squared analysis in SPSS?]

30. [:FAQ/RepeatedMeasuer:Repeated measures, Mixed models and Split-plot designs: A Rant]

31. [:FAQ/screen:How do I copy a SPSS window into a document?]
32. [:FAQ/emailSpo:How do I e-mail a SPSS output file?]
33. [:FAQ/pdfs:How do I convert a Microsoft file application (e.g. powerpoint) into a pdf file?]

34. [:FAQ/ExcelErrorBars:How do I put error bars on an EXCEL bar and line graphs?]

35. [:FAQ/points:How do I scatterplot observations which have the same set of co-ordinates?]
36. [:faq/excelplot:How can I distinguish groups on a scatterplot in Excel?]
37. [:FAQ/lines:How do I do a multiple line plot of cluster profiles for different factors in SPSS?]
38. [:FAQ/bar:How do I produce a bar chart with empty categories in SPSS?]

39. [:FAQ/BinomialConfidence:Improved Confidence Limits for a binomial proportion]

40. [:FAQ/barpercent:How do I produce an interactive bar chart of percentages in SPSS?]
41. [:FAQ/regline:How do I put a regression line on a scatterplot in SPSS Version 12.0 and above?]
42. [:FAQ/box:How do I produce a clustered boxplot and other more advanced graphics in SPSS?]
43. [:FAQ/ppts:How do I convert a powerpoint show file so it can print handouts?]
44. [:FAQ/skew:How should I deal with outliers when doing correlations?]
45. [:FAQ/partsp:How do I produce nonparametric Spearman partial correlations using SPSS?]

46. [:FAQ/Collinearity:What is Collinearity in multiple regression, and what do I do about it?]

47. [:FAQ/contint:How do I test for an interaction involving a continuous variable?]
48. [:FAQ/condnoR: How do I find the condition number of a matrix using R?]
49. [:FAQ/facnum: How do I convert a factor variable into a numeric in R?]
50. [:FAQ/nom: How do I perform a regression with a categorical outcome?]
51. [:FAQ/dummyCor:Regression diagnostics for categorical variables]

52. [:FAQ/CounterBalancing:How can I design an experiment so that conditions are counterbalanced for order?]

53. [:FAQ/wald:What is the Wald statistic?]
54. [:FAQ/x1se:How do I compute the standard error of X1 in a regression also featuring X2?]

55. [:FAQ/ResidualsNonNormality:Residuals and (Non-)Normality]

56. [:FAQ/ResidsRepMeas:How do I check Normality assumptions in repeated measures analyses in SPSS?]

57. [:FAQ/WsubEffects: What is the difference between within subjects effects and within subjects contrasts in SPSS?]

58. [:FAQ/MANOVA:When should I use a Multivariate Analysis of Variance (MANOVA)?]

59. [:FAQ/Simon:Testing normality including rules of thumb for skew, kurtosis in SPSS]

60. [:FAQ/kw:How do I compare a specific pair of groups post-hoc in SPSS using the Kruskal-Wallis test?]
61. [:FAQ/fried:Post-hoc nonparametric pairwise comparisons of a one-way within subjects factor]
62. [:FAQ/mwp:How do I know whether to use an exact or asymptotic p-value with a Mann-Whitney or Kruskal-Wallis test?]
63. [:FAQ/npreg:Is there such a thing as a nonparametric regression?]
64. [:FAQ/npexact:Performing randomisation tests using nonparametric methods]
65. [:FAQ/ranksum:What is the expected total discrepancy score in a R choice task?]
66. [:FAQ/pvs:How do I adjust p-values for number of comparisons using SPSS?]

67. [:FAQ/SpssBonferroni:What are adjusted p-values in SPSS?]

68. [:FAQ/CombiningPvalues:Combining p-values by Fisher's and Stouffer's methods]

69. [:FAQ/CombiningZandT:Summing z values and summing t values]

70. [:FAQ/Fdfs:Why does the value of one in the F distribution have a p-value which is less than 1?]

71. [:FAQ/BinomialConfidence:Improved Confidence Limits for a binomial proportion]

72. [:FAQ/RatCaseVar:Is there an optimal ratio of cases to predictor variables I should have before doing a multivariate analysis?]

73. [:FAQ/RegCor:What is the relationship between significance tests of regression coefficients and of correlation coefficients?]

74. [:FAQ/rsqdiff:How do I compare two squared correlation coefficients (R-squareds)?]
75. [:FAQ/multilevel: What are multilevel models and when do I use them?]
76. [:FAQ/subsets: How do I perform all possible subsets regression in SPSS?]

77. [:FAQ/AdequateFit:How well does a regression line fit?]

78. [:FAQ/dummy: How do I construct dummy variables for use in SPSS linear regression?]

79. [:FAQ/RegressionOutliers:Checking for outliers in regression]

80. [:FAQ/RegressionPart:What is the role of a Part or Semi-Partial Correlation in a regression?]

81. [:FAQ/avcor:How do I obtain an average correlation?]
82. [:FAQ/corrs:How do I test if a correlation is zero?]
83. [:FAQ/corrsCi:How do I obtain a 95% confidence interval for a correlation (or slope in a simple regression) in SPSS?]

84. [:FAQ/WilliamsSPSS:How do I compare a pair of correlations?]

85. [:FAQ/Power: Power calculations for testing if correlations equal zero]
86. [:FAQ/polychoric: What are polychoric correlations and how do I compute them and use in SPSS?]
87. [:FAQ/patternMatrix: Which matrix of loadings do I use doing a principal components extraction or non-PC analysis with a direct oblimin rotation when doing a factor analysis?]
88. [:FAQ/thresholds: What thresholds should I use for factor loading cut-offs?]
89. [:FAQ/structure:How do I assess the importance of variables in a Normal Discriminant analysis?]
90. [:FAQ/compare: What is the difference between principal components analysis, principal axis analysis and other factor extraction methods?]
91. [:FAQ/PlusFac: How do I interpret variables which load on more than one factor?]
92. [:FAQ/NoFactors: How many factors/components should I retain in a factor/principal components analysis?]
93. [:FAQ/Cronbach: A note on Cronbach's Alpha.]
94. [:FAQ/PlusFac: How do I interpret variables which load on more than one factor?]
95. [:FAQ/cancorr: What is canonical correlation and where can I use it?]
96. [:FAQ/mds: What is multidimensional scaling and how do I do it?]
97. [:FAQ/missing:How do I handle missing data in multivariate analyses in SPSS?]
98. [:FAQ/cc:How do I find complete cases in SPSS and R?]
99. [:FAQ/lag:How can I detect identical cases in SPSS without having an ID number?]
100. [:FAQ/EXponential: How do I produce truncated exponential random variables using MATLAB?]
101. [:FAQ/rand: How do I generate a random sample in EXCEL?]
102. [:FAQ/perc: How do I find out how many people have a score above a certain value?]
103. [:FAQ/percentile: How do I compute percentile thresholds for exponential data and use these in outlier detection?]
104. [:FAQ/kappa: Additional kappa statistic evaluation in SPSS]
105. [:FAQ/icc: What is an intraclass correlation and how do I use it?]
106. [:FAQ/rest: A note on correcting for restriction of ranges which underestimate Pearson correlations]
107. [:FAQ/rescore:How do I sum rescored scales in SPSS?]
108. [:FAQ/summiss:How do I obtain sums and means of partially complete cases in SPSS?]

109. [:FAQ/MeanSum:How do I obtain the mean of several variables, each minus the same constant?]

110. [:FAQ/surv:How do I adjust for age in comparing survival times of two different groups?]
111. [:FAQ/atan:Arctan functions in EXCEL.]
112. [:FAQ/cdfs: How do I obtain statistical distributions in MATLAB?]
113. [:FAQ/gamma:How do I produce random variables which follow a negative skew distribution?]
114. [:FAQ/boot: Simulations sampling from data with replacement (bootstrapping)]
115. [:FAQ/infmles:Why are the standard errors so large in logistic regression?]
116. [:FAQ/logrrsq:How do I choose between different logistic regression models?]
117. [:FAQ/OddsRa: Using an Odds Ratio to summarise a 2x2 frequency table]
118. [:FAQ/oddsr:How do I test an odds ratio?]
119. [:FAQ/threshold:Which discriminant analysis should I use to obtain thresholds to indicate levels of abnormality using a single variable?]
120. [:FAQ/roc:How do I plot and interpret a ROC curve in assessing strength in two group prediction?]
121. [:FAQ/criteria:Can you recommend any two group discriminant diagnostics?]
122. [:FAQ/sigdet:How do I compute signal detection diagnostics?]
123. [:FAQ/sem:Structural equation modelling in R]
124. [:FAQ/mahal:How do I detect multivariate outliers?]
125. [:FAQ/texts: Can you recommend any statistical textbooks?]
126. [:FAQ/pvalues:How do I find p-values using critical values as input in SPSS?]

127. [:FAQ/SpssBonferroni:Adjusted p-values]

128. [:FAQ/FDR:How do I do False Discovery Rate (FDR) corrections for multiple tests?]

129. [:FAQ/Bayes:How do I calculate and interpret conditional probabilities?]

130. [:FAQ/word:How do I produce equations using MS Word?]
131. [:FAQ/effectSize: A guide to magnitudes of effect size]
132. [:FAQ/dunlap: A quick guide to choice of sample sizes for Cohen's effect sizes]
133. [:FAQ/td: How do I convert a t-statistic into an effect size?]
134. [:FAQ/effcis: A guide to obtaining confidence intervals for effect sizes]
135. [:FAQ/Escomp: How do I compute effect sizes?]
136. [:FAQ/power:How do I do power calculations in SPSS, EXCEL and R?]
137. [:FAQ/equival:How do I compute statistical tests of equivalence?]

138. [:FAQ/SPSSpow:Computing Power for ANOVAs using the SPSS GLM procedure]

139. [:FAQ/JonckheereTrendTest Jonckheere's Trend Test]
140. [:FAQ/orthopolys:Fitting linear orthogonal polynomials]
141. [:FAQ/ss:Formulae for interaction sums of squares in balanced designs]

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These pages are maintained by [mailto:ian.nimmo-smith@mrc-cbu.cam.ac.uk Ian Nimmo-Smith] and [mailto:peter.watson@mrc-cbu.cam.ac.uk Peter Watson] (/center)

[Last updated on 1 July 2006]