Context:
I have two data sets from the same questionnaire run over two years. Each question is measured using a 5-Likert scale.
Q1: Coding scheme
At the moment, I have coded my responses on a [0, 1] interval, with 0 meaning "most negative response", 1 meaning "most positive response", and other responses spaced evenly between.
- What is the "best" coding scheme to use for the Likert scale?
I realise that this might be a bit subjective.
Q2: Significance across years
- What is the best way to determine whether there is statistically significant change across the two years?
That is, looking at the results for question 1 for each year, how do I tell if the difference between the 2011 result and the 2010 result is statistically significant? I've got a vague recollection of the Student's t-test being of use here, but I'm not sure.
Best Answer
1. Coding scheme
In terms of assessing statistical significance using a t-test, it is the relative distances between the scale points that matters. Thus, (0, 0.25, 0.5, 0.75, 1) is equivalent to (1, 2, 3, 4, 5). From my experience an equal distance coding scheme, such as those mentioned previously are the most common, and seem reasonable for Likert items. If you explore optimal scaling, you might be able to derive an alternative coding scheme.
2. Statistical test
The question of how to assess group differences on a Likert item has already been answered here.
The first issue is whether you can link observations across the two time points. It sounds like you had a different sample. This leads to a few options: