"Elle" <honda.lioness@gmail.com> wrote in message
news:4Ru0l.4052$%z5.3924@newsfe09.iad...
> "C. E. White" <cewhite3@removemindspring.com> wrote
>> "Lloyd" <lparker@emory.edu> wrote
>>> Why don't you READ CR's explanation of their analysis
>>> before spouting
>>> off and looking stupid?
>>
>> http://www.consumerreports.org/cro/c...ity-faq_ov.htm
>>
>> I have read it and I still believe the little circle over
>> emphasize small differences, that may not be statistically
>> valid. In the discussion they talk about the average
>> circle representing a failure rate of 2.5% (not an actual
>> value, but their example).
>
> In what part did you see this mention of 2.5 percent? I see
> it only in section 4.2, and it says nothing like what you
> say.

Yes, section 4.2. I expressed myself poorly. In the CR example of what
happens with small problem rates they use the 2.5% average failure
rate as an example (which is why I said "not an actual value, but
their example"). Although 2.5% might not be an actual average failure
rate, I think the information from other sources (like JD Power)
indicates that the average failure rate for new cars is very low,
definitely in that range. So assuming that the average failure rate is
very low, around 3% Consumer Reports slices the difference between a
3% failure rate a 0% failure rate into three categories (Excellent,
Very Good, Average). Given small sample sizes (as small as 100
responses), do you really believe the potential error rate for the
survey results support such fine divisions? I'd GUESS that the error
rate is probably on the order of +/- 5% for many of the vehicle with
smaller sample sizes (and by smaller , I mean in the several 100's of
responses). I think not. This is why I think CR's method over
emphasizes small, statistically insignificant differences in vehicles.

>> Do you think that a sample size of 100 vehicles is enough
>
> This is the minimum required sample size. If you read the
> site linked above, it notes that 200-400 cars per model year
> is usual.

And this is still a very small sample size when you are talking about
very small actual differences in failure rates.

> I disagree with other points you are making.

Which points are those? Do you think that I am wrong when I refer to
the people who respond to the CR surveys as "opinionated, motivated
people who like Consumer
Reports?" It seems to me this is given. The only question is whether
this group is typical of all car buyers. I think not. Do you think
they would be likely to strongly disagree with CR's editorial
opinions? I am sure some do (people like me), but I think most then to
be somewhat biased towards agreeing with CR.

Ed