Unipolar versus bipolar measures

The issue

Linked to the issue of scale anchoring is whether the scale is intended to be unipolar (i.e. reflecting a single construct running from low to high) or bipolar (running between two opposing constructs). In a unipolar format, the scale midpoint is intended to represent a moderate amount of the variable of interest, whereas in a bipolar format the midpoint is intended to represent a more neutral territory in between the two opposing constructs:

A unipolar scale:

A bipolar scale:

Although this distinction between bipolar and unipolar scales may seem very subtle, it should, in theory, have significant consequences for the meaning of the scale points. For example, a score of 0 on the unipolar scale above implies the absence of happiness, whereas a score of 5 implies a moderate amount of happiness. Conversely, on the bipolar scale, a score of 0 should denote complete un happiness, a score of 5 implies the respondent is neither happy nor unhappy, and a score around 7 or 8 would imply a moderate amount of happiness. If scale polarity - and the meaning of the midpoint value - is not completely clear to respondents, they may vary in how they interpret the scale, thus introducing a source of error. Furthermore, if one study adopts a 0-10 bipolar scale set of anchors, and the other a 0-10 unipolar set, mean values on these measures may not be comparable, despite both adopting 11-point response scales.