# Frequently Asked Questions

The FAQs below will be divided into categories including those above. Some of the FAQs have a few mathematical formulae which may be seen using mathplayer which may be downloaded for free from here. Computer code is featured in some of the FAQs and is highlighted in a (grey coloured) text box. This code can be copied directly to your personal PC file (e.g. into a SPSS syntax file) however you also need to copy any small piece of text immediately *outside* the text box when you copy the code in order to copy over the linebreaks or else you will find the text is copied over as a single line!

If you wish to search for a particular word and are using Internet Explorer just press Control-f and a 'find' slot will appear above the page in your browser. Also please note that macros in the EXCEL spreadsheets will not work (by default) unless they are enabled.

How many observations do I need to make z-scoring (assuming Normality) feasible?

What is the relationship between the z, t, chi-square and F distributions?

What is the probability of a repetition in a sequence of size k taken from K stimuli?

Why does SPSS give a row of asterisks instead of a mean in the outputted pivot table?

What does a number containing an 'E' signify and how do I remove it?

How do I check for outliers in a simple regression with one predictor variable?

How do I explain adding 0.5 to the cells for the odds ratio, d' (dprime) or logit transform?

What is heterogeneity of variance in SPSS Probit and Logit (and Poisson) Regressions?

What is overdispersion in handling proportions or count data and how do I handle it?

How do I perform tests of Marginal Homogeneity between two raters measuring the same items?

When should I use a logit analysis as opposed to an arcsine transformed ANOVA?

Why don't I see fiducial confidence intervals doing a probit analysis in version 16 of SPSS?

How do I do a matched pairs comparison on dichotomous data using covariates in SPSS?

How do I compare a mean to a constant in EXCEL (one-sample t)?

A note about different sums of squares in unbalanced factorial ANOVAs

A note on comparing confidence intervals and statistical significance

What is the variance of the mean of my transformed data and the variance of combinations of means?

What is the value of the error variance for raw data using that of its scalar transformed data?

A quick approximation for Normally distributed expected order statistics

How do I derive an expected total for a subset of items based on the expected overall total?

How do I format data for input into a repeated measures analysis in SPSS?

What does the TRANSPOSE ALL DATA option in the RESTRUCTURE menu do in SPSS?

How do I perform a repeated measures analysis of variance in SPSS?

How do I perform a repeated measures analysis of variance in R including correcting for sphericity?

How do I perform a repeated measures analysis of variance in GENSTAT (and MATLAB)?

How do I perform a non-standard comparison of means in a repeated measures anova in SPSS?

How do I test for an interaction involving a continuous variable (moderation analysis)?

How do I perform a stepwise regression in SPSS involving interactions?

Why don't I need to use a covariate which differs between randomised groups in an ANCOVA?

How do I handle errors in variables to estimate slopes and intercepts in a linear regression?

Inappropriate use of a constant covariate in repeated measures ANCOVA

How do I adjust for varying covariates in a repeated measures ANOVA in SPSS?

What summary measures can I use to describe repeated measures?

How do I obtain covariance terms involving the intercept term in a linear regression using SPSS?

Are there any primer publications explaining rationale and application of single case studies?

How do I compute linear trend coefficients for single cases?

How do I compare a within subjects group difference to that of a single case?

Why won't SPSS let me do post hoc tests involving a within-subjects factor?

How do I get SPSS to do a Chi-squared analysis of a two-way frequency table?

How do I compare a list of observed frequencies with a list of my own expected frequencies?

Using a chi-square to see if two or more proportions are equal

How do I know which elements contribute to a relationship in a two-way frequency table?

When do I use the correction for continuity when performing a chi-square analysis on a 2x2 table?

How do I do a linear trend of proportions in a Chi-squared analysis in SPSS?

How do I test for the presence of an unknown ordering across subjects using 2 Dimensional data?

Repeated measures, Mixed models and Split-plot designs: A Rant

Why do I get a NAME? appearing in cells when I try to run an EXCEL spreadsheet program?

How do I split comma delimited data occurring in a single cell into separate columns in EXCEL?

How do I make bibliographic citations of a SPSS or R procedure?

Why does SPSS give me a file definition error message when I run syntax?

How do I get my SPSS output file to open in a new version of SPSS?

How do I e-mail a SPSS data file created on a PC so that it is readable on a MAC (and vice-versa)?

How to avoid "$ operator is invalid for atomic vectors" in R

How do I convert a Microsoft file application (e.g. powerpoint) into a pdf file?

How do I scatterplot observations which have the same set of co-ordinates?

How do I do a multiple line plot of cluster profiles for different factors in SPSS?

How do I produce a bar chart including one with empty categories and other features in SPSS?

Improved Confidence Limits for a binomial proportion and differences in binomial proportions

How do I compare observed numbers correct to those expected by chance in a multi-choice task?

How do I produce an interactive bar chart of percentages in SPSS?

How do I put one or more regression lines on a scatterplot in SPSS Version 12.0 and above and in R?

How do I plot user defined regression lines (such as those from ANCOVA) in R?

How do I produce a clustered boxplot and other more advanced graphics in SPSS?

How do I convert a pdf file into a MSWord or MSPowerpoint file?

How do I convert a powerpoint show file so it can print handouts?

How do I add in the analysis toolkit (and other add-ins) into EXCEL?

Rank-based and other correlations (percentage bend) which are robust to non-Normal data

Can I do a correlation between two variables where one variable is less than or equal to the other?

How do I produce nonparametric Spearman partial correlations using SPSS?

What is Collinearity in multiple regression, and what do I do about it?

How do I look for the best fitting model with an unknown changepoints on ordered data?

How can I design an experiment so that conditions are counterbalanced for order?

How do I compute the standard error of X1 in a regression also featuring X2?

How do I test to see if a mean adjusted for a covariate equals zero in a single group?

How do I compute the standard error of ''beta'' in a linear regression in SPSS?

How do I check Normality assumptions in repeated measures analyses in SPSS?

What is the difference between within subjects effects and within subjects contrasts in SPSS?

When should I use a Multivariate Analysis of Variance (MANOVA)?

Testing normality including rules of thumb for skew, kurtosis in SPSS

How do I compare a specific pair of groups post-hoc in SPSS using the Kruskal-Wallis test?

Post-hoc nonparametric pairwise comparisons of a one-way within subjects factor

What is the expected total discrepancy score in a R choice task?

How do I adjust p-values for number of comparisons using SPSS and R?

Why does the value of one in the F distribution have a p-value which is less than 1?

How do I compute the semi-partial correlation coefficient in R?

How do I compare two squared (semi-partial) correlation coefficients from the same sample?

How do I adjust R-squareds for the number of predictors in a model?

What are random effect and multilevel models, when do I use them and are there effect sizes?

What is a Generalized Additive (Mixed) Model (GA(M)M) and when do I use it?

How do I construct dummy variables for use in SPSS linear regression?

What is the role of a Part or Semi-Partial Correlation in a regression?

How do I test if a correlation is zero and compute its confidence interval?

How do I estimate a pooled correlation using multiple scores from a set of subjects?

How do I adjust p-values to test if more than one correlation is zero?

How do I adjust a correlation for group differences (using partial/semi-partial correlations)?

When and how do I evaluate a one-sided p-value and quote a one-sided 95% confidence interval?

What sample sizes do I need for doing tests with a given power?

What are polychoric correlations and how do I compute them and use in SPSS?

A guide to the pros and cons of choosing a method for producing factor scores from a factor analysis

How do I assess the importance of variables in a Normal Discriminant analysis?

How do I interpret variables which load on more than one factor?

How many factors/components should I retain in a factor/principal components analysis?

How do I interpret variables which load on more than one factor?

How do I handle missing data in multivariate analyses in SPSS?

Using SPSS syntax to impute last observation carried forward (LOCF) for missing values in SPSS

How can I detect identical cases (duplicates) in SPSS without having an ID number?

How do I produce truncated exponential random variables using MATLAB?

How do I obtain parameter estimates for finite Normal mixture distributions using SPSS and BMDP?

How do I find out how many people have a score above a certain value?

How do I compute percentile thresholds for exponential data and use these in outlier detection?

A note on correcting for restriction of ranges which underestimate Pearson correlations

How do I obtain sums and means of partially complete cases in SPSS?

How do I obtain the mean of several variables, each minus the same constant?

How do I compute z-scores in SPSS and what is their relationship to comparing group means?

How do I use cumulative distribution functions to compute p-values in SPSS, EXCEL and R?

How do I adjust for age in comparing survival times of two different groups?

What is the relationship between intercept and slope for scores at two time points?

How do I produce random variables which follow a negative skew distribution?

Simulations sampling from data with replacement (bootstrapping)

Generating multivariate data with a required correlation matrix

How do I obtain the formulae behind statistics outputted by SPSS algorithms?

How do I compute a leaving-one-out error rate for a logistic regression in SPSS?

Why are the standard errors so large in logistic regression?

How do I choose between different logistic regression models?

How do I interpret output from a Multinomial logistic regression?

How do I plot and interpret a ROC curve in assessing strength in two group prediction?

What do I do if I have unequal group covariance matrices when doing a MANOVA?

How do I find p-values using critical values as input in SPSS?

How do I do False Discovery Rate (FDR) corrections for multiple tests?

A quick guide to choice of sample sizes for Cohen's effect sizes

How do I convert a t-statistic (and an Odds Ratio) into an effect size?

How do I compute effect sizes (including variance adjusted ones)?

How do I do power calculations in SPSS, EXCEL, R and using web freeware?

How do I do power (sample size) calculations on Poisson counts?

How do I work out sample size for apriori specificities and sensitivities?

How do I compare group means in a non-standard post-hoc contrast?

How do I compare a set of group means with a control group mean?

Formulae for interaction sums of squares in balanced designs

Return to Statistics main page

These pages are maintained by Ian Nimmo-Smith and Peter Watson (/center)

[Last updated on 1 July 2006]