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Using data collected at several sites to study seed germination in
Downy Brome (a weed in the grass family), several models are fit to
provide answers to the following questions: (1) At what value of {\em
growing degree days (GDD)}, a temperature/time metric, does seed
germination become non-zero? (2) After germination is non-zero, what
is the relationship between GDD and germination? A first stab at
answering these questions is a manual, two-step approach in which the
first question is addressed, and under its solution the data are
reduced to include only non-zero germination samples, wherein the
second question is addressed. A second approach automates these
two-steps using a {\em change-point model}---the GDD value at which
germination becomes non-zero is estimated, and a linear model is fit
to the non-zero data. From there, more complex (more realistic?)
Bayesian hierarchical change-point models are considered to account for
differences across sites. Data and motivational questions for this
exploration were provided by Dan Ball, Associate Professor of Weed
Science with the Columbia Basin Agricultural Research Center in
Pendleton, Oregon.
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