I've come across a paper (mainly written in Spanish, which I don't speak - but let's not stop there) which points out that there is no agreed conversion factor between levodopa and other PD drugs.

For each drug and each method of calculating the equivalence Cervantes-Arriagal et al. [1] give the conversion factor. I precis their table as follows:

Drug/Lowest conversion factor/Highest conversion factor/Weighted average
L-dopa/1/1/1
L-dopa CR/0.70/0.77/0.74
Pergolide/100/100/100
Cabergoline/66.6/100/80.12
Bromocriptine/10/10/10
Stalevo/1.2/1.33/1.25
Pramipexol/67/100/89
Ropinirole/16.67/33.3/21.3
Lisuride/100/100/100
Apomorphine/8/10/8.25

(N.B. the l-dopa entry is for l-dopa with carbidopa.)

What sticks out to me is the large range for ropinirole: a factor of two difference between the highest and lowest estimates. Also, so many of the estimates are "round numbers", such as 100, which suggests that the estimates are very rough. That said, I think it is very useful to have an idea of the relative power of drugs.

Reference

[1] "Cálculo de unidades de equivalencia de levodopa en enfermedad de Parkinson"
Amin Cervantes-Arriaga1, Mayela Rodríguez-Violante1, Alejandra Villar-Velarde, Teresa Corona
Arch Neurocien (Mex) Vol. 14, No. 2: 116-119; 2009
http://www.medigraphic.com/pdfs/arcn...09/ane092f.pdf

John