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blaming the cultivar

When I said "iris growers as a group seem irrationally determined to
blame the cultivar", I seem to have got some backs up. It was not
intended as a personal remark, and I apologize to anyone who took it
that way.

But let me explain in a little more detail what I'm talking about,
and why there is a danger in jumping to conclusions based on observing
an iris in the garden for a few years.

Let's assume you have ten cultivars, which I'll label A through J.
Further assume that there is some problem--multiple petals, bloomout,
rot, aphid infestation: pick your favorite--to which irises are
susceptible. Imagine now that each iris has a 50% chance of getting
the "scourge" (whatever it is) each season. It's a completely random
phenomenon, no cultivar is any more susceptible than any other.

To do a thought experiment, I'll just flip a coin for each iris,
and repeat the process for three years. I'll jot down a "*" if the
toss comes up tails, meaning the iris gets the scourge. Here I go:

[This will look best with a proportional font.]

cultivar: A  B  C  D  E  F  G  H  I  J
year: 1   *  *  *     *  *  *  *     *
      2         *           *  *  *  *
      3   *  *  *        *     *

Looking at this, I see D never got scourged, C and H got scourged
every year, E and I got it only once, and the others all got hit twice.
As an iris grower, wouldn't you be VERY tempted to go to your local
club and sing the praises of cultivar D--and possibly slander C and H?

Yet there would be no basis for those opinions. The scourge simply
hit your iris bed at random each year.

Now suppose susceptibility to the scourge is genetically influenced--
some irises are especially scourge-prone, others are somewhat resistant.
Of course it's unlikely that the genes would make any iris 100% immune
(or 100% susceptible), so let's just say that the "resistant" varieties
get the scourge 30% of the time, and the "scourge-prone" varieties get
it 70% of the time. I'll now pick three of the ten to be resistant, and
three to be susceptible. But I'll keep my choices secret (for now),
and run the experiment again:

cultivar: A  B  C  D  E  F  G  H  I  J
year: 1         *           *  *
      2         *  *     *  *        *
      3            *           *  *  *

Can you guess which ones are susceptible and which are resistant?
You'd probably put A, B, and E in the resistant group, and C, D,
G, H, and J in the susceptible group, right?

It turns out I chose A, B, and C as the resistant ones, and H, I,
and J as the susceptible ones. So with the judgment from the
previous paragraph, you would have been right about only 4 out of
7 irises (about 57%). Remarkably, C would be branded as scourge-prone
even though it is in fact resistant!

The point of this little thought experiment is to illustrate how
the human mind has a propensity to draw conclusions from insufficient
evidence. When I mention the horrible word "statistics", it's not
just because I'm a nerd who likes to ruin the fun by talking about math.
it's because I'm aware of the dangers of relying on anecdotal evidence.

If the first example above had been a real-life record of what had
happened in someone's garden (not just a thought experiment), my caution
that "scourge may just effect irises at random" would surely be met with
"But I grew D for three years and it NEVER got it, while C and H were
wiped out every year three years running--explain THAT!"


I hope this clarifies the point. I really do not mean to be argumentative,
just to educate a bit.

Happy irising, Tom.


Tom Tadfor Little         tlittle@lanl.gov  -or-  telp@Rt66.com
technical writer/editor   Los Alamos National Laboratory
Telperion Productions     http://www.rt66.com/~telp/

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