Designing Likert scales

An obvious but essential first recommendation is to thoroughly review the existing literature on the construct of interest. This is necessary to develop a good definition, clearly positioned within the existing literature and related to what we are hoping to assess (Artino et al., 2014). For example, if you would like to design a new scale to measure self-esteem, it is wise to review the literature and existing self-esteem scales to understand how it relates to your research question. This preliminary research can avoid misconceptions, like the idea that high self-esteem improves academic performance which is discredited in the academic literature (Baumeister et al., 2003).

Survey designers should also search how the construct of interest has been measured in the past. It can sometimes be more efficient and reliable to adapt existing scales or items than starting a new questionnaire from scratch. This approach also increases consistency between studies and thus simplifies comparisons. Knowing that, the specific context, sample and purpose of each specific study has to be taken into account.

Each item on a scale is the representation of one part of the construct of interest. For example, the item – ‘on the whole, I am satisfied with myself’ – is one part of the construct self-esteem on the Rosenberg Self Esteem Scale (Rosenberg, 1965).

When drafting items, survey designers have to stay close to the theory and put themselves in respondents’ shoes. Their main objectives should be simplifying language and minimising ambiguity. In practice, this means:

• Avoiding technical language (e.g., acronyms) and complex grammatical constructions that respondents might not understand quickly or accurately;
• Avoiding ‘double-barrelled questions’ (i.e., questions using conjunctions such as and/or/but) where respondents are asked to answer two different questions in one, making it impossible for the researcher to know which one they are actually answering.

Even if they follow these principles, survey designers might still fail to detect misalignments between their intention and respondent’s interpretation. Cognitive interviewing can solve these issues by testing whether respondents correctly and consistently interpret the Likert scale. In practice, interviewees are asked to describe their thinking as they answer each question or after they complete the questionnaire (Peterson et al., 2017).

Survey designers should keep in mind that there are practical limitations to the number of items that should be included in a questionnaire. Respondents have a limited amount of time and cognitive resources allocated to answer a survey. Adding too many items can therefore decrease response rates and survey reliability.

That is why item selection is a crucial and challenging step in survey design. Nemoto & Beglar (2014) recommend piloting about ten to twelve items, and selecting the best performing six to eight. Other researchers indicate that the ‘sweet spot’ is around five items per construct (Weng, 2004; Nielsen et al., 2017).

Respondents have a tendency to agree with declarative statements regardless of their content, a phenomenon referred to as acquiescence bias (Wright, 1975). There are ways to avoid acquiescence bias.

First, when selecting the items to include, survey designers should ensure that the scale’s entire response range will be used to express a certain opinion to minimise the risk that this bias affects results. For example, for a respondent to be identified as having low or high self-esteem, they should have to answer positively to some items and negatively to others.

Additionally, it is preferable to present the items as even-handed questions instead of statements and to avoid ‘agree-disagree’ response options (Fowler 2009, Saris et al., 2010). Instead of asking respondents to rate their level of agreement, using even-handed questions requires verbally labelled response options that reinforce the underlying topic.

For example, it is preferable to use ‘Are you satisfied with yourself?’ rather than ‘Overall, I am satisfied with myself’. A suitable set of response labels could be: not at all satisfied, slightly satisfied, somewhat satisfied, quite satisfied and extremely satisfied.

For many years, researchers recommended that survey designers include both negatively and positively worded items in questionnaires (Nunnally, 1978; Spector, 1992). However, more recent studies have found that negatively worded items might create more validity and reliability issues than they solve, for various statistical and behavioural reasons (Allen, 2017).

For instance, the negative expression of a statement requires more complex cognitive processes, making it more challenging for respondents with lower reading abilities or general education to provide consistent responses (Swain et al., 2008).

Hence, evidence suggests that survey designers should phrase items with positive language. They can adopt alternative techniques to minimise biases and improve respondents’ attention. For instance, Instructional Manipulation Checks, items asking respondents to demonstrate that they are paying attention by choosing a particular response option (e.g., “Select ‘Strongly disagree’ if you are reading the items and paying attention”) can be added in Likert scales (Berinsky et al., 2014). Another technique consists of changing the direction of the response options for each few items (i.e., alternating between the positive to negative direction from left to right, and the negative to positive direction).

There is an ongoing debate regarding whether a midpoint should be included in Likert scales (Croasmun & Ostrom, 2011). This midpoint is commonly labelled as ‘Undecided’, ‘Neither agree nor disagree’ or ‘Don’t know’. Naturally, including or excluding a midpoint makes the scale an odd-numbered or even-numbered scale.

There are statistical and behavioural arguments on each side. For instance, a midpoint can allow respondents to express indecision or neutrality (Johns, 2005) but might be misused (e.g., as an alternative to avoid reporting a potentially controversial opinion), whilst its absence can artificially force respondents to commit to a certain position.

Hence, the question that practitioners should seek to answer is not whether or not to include a midpoint, but rather when to add it or omit it. Chyung et al. (2017) present some evidence-based recommendations on the topic. On the one hand, they recommend including a midpoint when:

• Respondents are familiar with the topic and should be allowed to express a neutral opinion;
• The likert scale has to be used as an interval scale for statistical analysis purposes.

On the other hand, a midpoint should be omitted when:

• Respondents are unfamiliar or uncomfortable with the survey topic;
• Respondents are not expected to have formed their opinion about the topic;
• Social desirability bias may have a strong influence on responses;
• Respondents are likely to put little effort into answering the survey.

More generally, this decision should be made by taking into account factors such as scale sensitivity (i.e., smallest absolute amount of change that can be detected by a measurement), age and education level of respondents, choice of the midpoint label and researcher’s preference (Chyung et al., 2017).

An anchor is a word or sentence that describes a position on a response scale. Research has shown that, on a Likert scale, it is best to use as many verbal labels as possible. For instance, for the question ‘How would you rate your mental health overall?’, the anchor points could be : excellent, very good, good, fair, bad and very bad. It is particularly important in order to discourage respondents from becoming reliant on the midpoint when responding to a scale (Matell & Jacoby, 1972). We thus recommend labeling each anchor point to maximize data quality by reducing measurement error (Artino & Gehlbach, 2012). Thankfully, it is also what respondents prefer (Johns, 2010)!

Closely related is the debate around the number of points to include on a Likert scale. Although Likert originally designed five-point scales, there are no theoretical reasons to rule out different numbers.

Scales with fewer points are easier and quicker to use but do not provide as much information (Preston and Colman, 2000). Larger scales can be harder for the respondents to interpret but provide researchers with more nuanced and accurate information about the repondants’ answers.

While researchers have not agreed on a single optimal number of points to include on a Likert scale, most recommend between five and seven points (Preston & Colman, 2000; Chen and al., 2015). The decision is dependent upon the characteristics of the study, particularly the cognitive capacities of its respondents. For instance, Weijters et al. (2010) recommend using five-point scales for the general population, and seven-point scales for populations with relatively high levels of verbal skills and experience with questionnaires. Nemoto and Beglar (2013) encourage the use of four-point scales for young respondents.

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