Hammilton,
You are absolutely correct: practice does improve your score.
The quantification of this learning effect is one of the objectives of an online survey and test that I'm running at:
http://www.parkinsonsmeasurement.org/PDMeasure
So far, 90 people have done the test a total of 200 times.
You're very welcome to take part.
It would be interesting to know whether all clinical trials make a large enough allowance for the learning effect.
John