Following grammar rules I know (quoting):
The ’s is placed immediately after the last letter of the author (or name, country, etc.). Note: do not use the before the name of the author.
Do not use the + name of person + ’s.
So according to that, the following example is wrong:
The Simpson’s paper is an excellent introduction to the topic
But why the following works:
The Parkinson's Foundation makes life better for people with Parkinson's disease
The coalition has developed the Parkinson's Disease Composite Scale
I feel the definite article is somehow bound to the "foundation" and "composite scale" words in the examples. But on the other hand, it is before the name (Parkinson) which uses the genitive.
Or in this case, Parkinson's is just understood as the name of the disease and thus it is used as an adjedctive? As in The Clinton foundation (why not Clinton's, then).