Medication for Parkinson's is, albeit after some delay, often directly felt by a PwP. This is especially true of levodopa-carbidopa because of both levodopa's effectiveness at reducing the symptoms of PD and its short half-life. This makes the actual, as opposed to the nominal, size of the dose important for PwP.

Although general in its coverage, and not being specifically about Parkinson's, a paper by Paveliu et al. raises some interesting issues.

Batch/batch variance: Pavelui writes [1]
"Various pharmacopoeia admit variations of + / -5% margin in active ingredient between the products [from] the same batch".

Brand/generic: based on work done by the FDA back in 1999:
"The observed mean difference between the innovator's product and the generic product for AUC (0-t) was ± 3.47% (SD, 2.84), for AUC (0-Inf) it was ±3.25% (SD, 2.97), and for Cmax it was ± 4.29% (SD, 3.72). ... So the concerns seem to be unjustified."
I think we should be concerned by a 4% average difference, especially because the actual difference could be higher.

Generic/generic variance. The FDA requirement for a generic is that the 90% confidence interval for its values of certain pharmacokinetic parameters lie within a range of 80% to 125% of the original. Note: this is not the same as saying that 80% or 125% is acceptable. This is because there will always be some variability leading to the confidence interval not having zero width. But let me make a gross approximation and say that you could get two generics, one with 90% of the pharmacokinetics of the original and another one with 110% of the original. So someone stepping from one generic to another, different generic could face a 20 percentage point difference.

Reference

[1] "Generic Substitution Issues: Brand-generic Substitution, Generic-generic Substitution, and Generic Substitution of Narrow Therapeutic Index (NTI)/Critical Dose Drugs"
Marian Sorin PAVELIU, MD, PhD,a Simona BENGEA, MD, PhD,a and Fraga Silvia PAVELIU, MD, PhDa
Maedica (Buchar) Jan, 2011
Generic Substitution Issues: Brand-generic Substitution, Generic-generic Substitution, and Generic Substitution of Narrow Therapeutic Index (NTI)/Critical Dose Drugs

John