A little while ago I got a query about the calculation of the logit policy scales from Lowe et al. (2011). I thought it might be useful to repeat the answer slightly more publicly, in case anybody else was wondering. The pesky constants in that paper confuse people. Anyway, here’s the question:

In the article you give the formula as log(R+.5)-log(L+.5). I had assumed that in the formula that ‘R’ and ‘L’ were the total number of sentences on each ‘side’ of a policy scale and so consequently .5 is added to the total number of sentences in all the manifesto categories assigned to each side of a policy scale. However I was reading an article [… where they] seem to add .5 to each of the categories assigned to a policy scale and then also divide by the number of items used in the scale (their approximate formula [without proper subscripting] is: p = [(log(p_1 +0.5)-log(p_2+.5)+ \ldots +(log(p_3+.5)-log(p_4+.5)]/3) where p is a manifesto category). Consequently I’m slightly worried that I’ve misinterpreted how you calculate your scale

OK, so the way to think about this scale is as follows…

Continue reading Constants in Logit scales