Post by SMSPost by Michael PardeeGetting a random sample is invariably the biggest challenge to any
survey, and CR doesn't even pretend to try. Self-selection - even when it
is more subtle than CR's open invitation format - will turn any survey to
trash. Many a survey has been invalidated by the mere fact that in most
cases the potential respondents can't be compelled to respond, and can't
be compelled to respond honestly. No quantity of self-selected
respondents can produce usable results. And that is the case with CR's
surveys.
You're confusing "usable results" with the "perfect results" of a
double-blind random survey.
Unless you believe that a large number of the subscribers that respond to
the survey are intentionally lying only about certain vehicles, while
telling the truth about others, the reliability survey is in fact very
usable. You have to take it for what it is, a survey of owner's
experiences of the reliability of their vehicles, with results only
reported if a sufficient number of respondents own the vehicle in
question. If 30% of Camry owners report problems with the transmission,
and 5% of Accord or Taurus owners report similar problems, you have some
usable information. Maybe a double-blind random survey would have slightly
different percentages, but the information in the CR survey is still
valid.
It doesn't matter what the survey is, or what the source is, you always
have people that don't like the results trying to attack it if it's not a
double blind random survey. You see the same thing with the CR survey on
cellular carriers, the largest survey of its kind. Again, you have a few
subscribers of the carriers that do extremely poorly year after year
whining that the perception of Verizon of a carrier with superior coverage
is causing the Verizon subscribers to rate it highly, while the AT&T
subscribers somehow are out to bash AT&T. This is despite the fact that
every other survey from non-advertiser based organizations reports the
same results.
Then you have the people that are confused about quality versus quantity,
claiming that since McDonald's sell the most hamburgers of any restaurant,
that proves that McDonald's has the best hamburgers, and that since GM
sells more vehicles than Honda or Toyota in the U.S., that proves that GM
produces the best vehicles.
Double blind techniques are used to prevent reporting and collection bias
when
the sample has already been selected, and that doesn't apply here.
Randomness
is important here, and that is where CR's surveys are most seriously
deficient.
Nor does it matter who complains about what survey; the real problems remain
with CR's methodology.
In your example of transmission problems, we can see how that works. If ten
percent of Camry owners are interested in responding to the survey, and of
those 30% are interested because they had transmission problems while the
rest are interested because they loved their car so much or they had some
other complaint, the result would be to magnify the transmission problem
from as little as 3% in the real world to 30% in the survey results because
30% of respondents (not owners) complained about their transmissions.
Conversely, if 90% of Taurus owners felt compelled to respond because there
were daggers embedded in the driver's seat (seating comfort = poor) but only
5% of those had transmission problems, the survey results would be as you
describe... although the reality would be that the Camry transmission
failure rate was as little as half the Taurus rate rather than six times as
high. This is an example of ascertainment bias
http://en.wikipedia.org/wiki/Ascertainment_bias
We can't even say what the actual rate was for Camrys even though we
know - as CR does not - only 10% of Camry owners responded, because we don't
know what the other 90% experienced. Did they have no complaints at all, or
did a lot of them have transmission trouble and were tired of dealing with
it? As long as we don't know why the responses were sent and the rate
of response for the various models there is no valid data to be had.
Check out http://en.wikipedia.org/wiki/Selection_bias and
http://en.wikipedia.org/wiki/Self-selection for more. In the first link, the
heading "Overcoming selection bias" warns, "In the general case, selection
biases cannot be overcome with statistical analysis of existing data
alone..."
Anyway, to each his own. When I have occasionally looked at CR's predictions
for cars I have owned I have been struck by how far off base they usually
were (although they got pretty close to my experience once!) And to give
them their due, when a problem is as pervasive as the AC and tranny failures
were for the 1993 and 1994 Volvo 850s they picked up on that. If you want to
credit them you can. Personally, I regard the CR surveys as not worth my
attention. I have access to a dartboard that is as reliable.
Mike