School of Veterinary Medicine and Science

Stats Club: Stats Club:

Ordinal data and likert scalesOrdinal data and likert scales

Dr Nigel KendallDr Nigel Kendall

School of Veterinary Medicine and Science

Content• What is ordinal data• Approaches for ordinal data

– Chi-square for trend– (Ordinal) logistic regression– Normal distribution statistics?– Non parametrics

• What is a likert scale– Is it ordinal data

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Chi-square for trend

• Normal chi-square just looks to see if groupings are random– Nominal / categorical data

• Chi-square for trend looks for a shift along an ordinal (or discrete ratio) scale– Appropriate for 2xc contingency tables (c>2)– Partition chi-square appropriate to a trend and

departure from that trend– Relatively complex equation (excel template)

Daya (2001) Chi-square test for trend (2xc contingency table) Evidence-Based Obstetrics and Gynaecology 1: 116-7

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(Ordinal) logistic regression(Ordinal) logistic regression

• Logistic regression can be considered• Allows you to look at the effects (and

interactions) of many factors• MINITAB has a specific ordinal logistic

regression

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Normal distribution Normal distribution statistics?statistics?

• Should always be considered• Remember it is the residuals from the

analysis which requires to be approximately normally distributed, not the variable itself

• Very robust to violations of assumptions• Transformations can be used

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Non parametricsNon parametrics

• Mann-Whitney U– Test basically ranks all values in a line and counts

number of a variable before each of the other variables and a p value is assigned

– Therefore can be used to give an indication of ordinal shift similar to chi-square for trend

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Likert scalesVarious types• Point types

– Strongly disagree– Disagree– Neutral– Agree– Strongly agree

• Line (scale) types

never always10cm

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Is it ordinal data?Is it ordinal data?

• Point data – yes potentially ordinal

• True scales – usually fit ANOVA type analysis– Therefore appropriate to quote mean and s.d.

Carifo and Perla (2008) Resolving the 50-year debate around using and misusing Likert scales. Medical Education 42: 1150-2