But if this is true, shouldn't the coding scale run -2, -1, 0, +1, +2 ?

Schwartz, N. (1999). Self-Reports: How the Questions Shape the Answers. American Psychologist, 54, 93-105.

You are assuming that the categories correspond to equal-size intervals. This is contested.

Unfortunately, the interpretation of the relative sizes of the intervals is an empirical matter. How can we get at that?

Unfortunately, the interpretation of the relative sizes of the intervals is an empirical matter. How can we get at that?

Unfortunately, the interpretation of the relative sizes of the intervals is an empirical matter. How can we get at that?

Bieri, D., Reeve, R. A., Champion, G. D., Addicoat, L., & Ziegler, J. B. (1990). The faces pain scale for the self-assessment of the severity of pain experienced by children: Development, initial validation, and preliminary investigation for ratio scale properties. Pain, 41(2), 139–150. doi:10.1016/0304-3959(90)90018-9

Would be interesting to hear what people think of this. One complication is figuring out what exactly we'd want the results to look like before we'd say the scale has "interval" properties. All participants placing all responses exactly equidistantly apart? All responses equidistantly apart on average? No significant differences between average distances between options?

Also, just in terms of the negative numbers - I think there's some kind of idea out there that the average populace finds negative numbers confusing and scary.

For good reason too. If you add a negative number to a positive number then the negative number kidnaps the positive number and neither are ever seen again - almost like neither ever existed. It's terribly tragic.

Perhaps the way to resolve my unease about the scale numbers is to eliminate the scale. I could be a traditionalist about Likert and just use the categorical descriptors so far as my survey form is concerned. What I do in my private data lair stays in my private data lair.

Thanks to all.

Terribly so. Not to mention the vile sorcery that's rumoured to go on when two negative numbers are multiplied together... a negative times a negative equals a positive?!! Mathematicians, they so crazy.