Robert
The point you make is sound. It is discussed before on the list. One does not
need much knowledge about mathematics to understand why. I have written
about it to the list after Ivan's survey on the relation of diabetes and
parkinson. We know about these two diseases that they have both a high
frequency in the population. One can knowing the frequency of both and
supposing they to are independent calculate in what percentage we are
supposed to see both diseases in the same patient. Which in this paradigm
means that if the number of diabetes is for people with parkinson the same
as for people without parrkinson. This seems clear But in doing research
one does not examine a whole population but a sample. One thing that we do
know is that human tend to see a causal relationship, if two things with an
unknown cause are happening in a certain time sequence. So one can expect
that a person who happen to have diabetes and after starting to take
insuline, will get symptoms of parkinsons disease will tend to believe the
insuline caused the parkinson. If one asks wether that could be true and
has the idea not been taken seriously one will stop talking about it. But
everyone can understand his reaction on an article in a paper or listserv
which mentions the relation beteen the two diseases and whose author is
convinced that relation exists and wants to make that an approved fact will
be quite different from that of people who have only one or none of both
conditions. part of this reaction will be they are more prone to answer the
survey. And so the survey will show a greater number of people with both
diseases as could be expected only only from the respective frequencies of
both.
To my opinion this difficulty could be solved by having a survey with all
known symptoms of parkinson. Having this answered by a husdred people, one
can not really prove anything, but relations between symptoms and patterns
of symptoms, found in this way are not without any meaning.
I don't know wether this is done allready. If someone knows, i'd like to
hear.
Ida Kamphuis53\13
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Mary,
>
> I responded to your survey as soon as I read it because it took me only a
> few minutes. I did it, however, even though surveys of this kind are
> usually not reliable. It is not that the questions you asked were
> deficient in any way, and I believe you would be objective in evaluating
> the answers.
>
> It is because retrospective studies are usually not a reliable way to
> establish correlations. I say usually because if everyone on the list
> answered and gave the same answer, you would indeed have uncovered
> something of value. But, results seldom turn out that way. Usually the
> correlations one might observe in retrospective studies have no validity
> and are useful only as a suggestion of one thing to look for in a more
> reliable subsequent study.
>
> This point was made an article by William Feller in the throwaway
> periodical 'Scientific Research' many years ago. I remember it well
> because it impressed me so much. Feller is the author of the book for a
> course on probability that I took. He was the first to present the subject
> in the now accepted way. The book went through a number of printings. He
> is well respected.
>
> His article in 'Scientific Research' was short and to the point. He
> probably chose that magazine because he wanted as many people as possible
> to get the message, which no doubt is widely known by probability theorists
> and mathematical statisticians, but seems frequently to be ignored by
> epidemiologists.
>
> Feller said that if you consider 100 uncorrelated random variables
> generated by a computer and calculated the correlations between all pairs
> of them, you would find at least one pair with a convincingly large
> numerical correlation. I do not recall the exact number, but I think it is
> something like 25 per cent or more, maybe even 50 per cent. Statisticians
> would say that value of the numerical correlation is 'statistically
> significant.' There are also a surprisingly large number of pairs for
> which the calculated correlations are large enough that someone who didn't
> know the source of the data might say that there is convincing evidence of
> a correlation. Again I do not recall the exact numbers, but I think they
> are in the 5 percent to 10 percent range. All this for random numbers with
> absolutely no real (i.e. causal) correlation.
>
> I am not an expert in statistics, and so I do not have definitive
> information to give you about this. I certainly am not skilled in
> designing a careful study. But I think Feller's point leads one to be
> cautious about the interpretation of retrospective studies.
>
> Baldwin, 63/4
> 8x25/100 carbi/levo-dopa, 5 mg Eldepryl per day
> [log in to unmask]
>
>
> ----------
> From: Mary Sheehan
> Sent: Thursday, October 23, 1997 11:06 PM
> To: Multiple recipients of list PARKINSN
> Subject: Response to PD Survey
>
> My PD started 11 years ago with a frozen shoulder. Bradykinesia and
> micrographia followed, but the doctors attributed my symptoms to the
> effects
> of the frozen shoulder. It wasn't until 21 months ago that I was diagnosed
> with PD. After 11 years with PD, I am still in stage 1. I take only 5mg of
> Eldepryl daily and have recently started to take Mirapex.
>
> I am telling you this so you will understand my motivation for doing the
> survey. I have been amazed at the different rates at which PD progresses,
> the different symptoms in each person and the different ways that each
> person is medicated. I came up with a premise that the progression and
> symptoms of PD must relate to either personal lifestyle or to how PD
> presents
> in an individual .... or possibly to a combination of both. I had hoped to
> use the resources of this list to find a pattern that made the progression
> of
> PD more predictable. I have done some successful research in the past,
> although not in the medical field, and felt there were clues out there
> waiting to be uncovered.
>
> Unfortunately, with only 18 responses, it was impossible to draw any
> conclusions. I had hoped for 100 responses, or more, out of the 1500
> people
> on the list. What really upset me was the responses i received as to why
> people didn't respond.
> " I'm too busy to reply". The whole survey couldn't have taken more than 5
> minutes to fill out. Most questions required only one-word answers.
> "I'm tired of all the surveys on this list." In the year and a half I've
> been on this list I recall only two persons doing a survey. I answer
> surveys
> in the small hope that maybe something will come out of it that will help.
> "The PD organizations can"t get together". So let them do their research
> separately. I have great respect for the researchers at the Parkinson's
> Institute and for my neuro who also does research and whom I thought would
> be
> interested in what we had to say.
>
> I'm sorry to be so wordy. I put in several hours of work on the survey and
> was annoyed to find my efforts denigrated. I'm probably only having a bad
> day, but I'm disappointed because I honestly thought we might put our heads
> together and discover something. I will shut up now and in the future will
> avoid initiating surveys . Thanks for letting me vent.
>
> Mary
>
