A Closer Look at Regional Curve Standardization of Tree-Ring Records: Justification of the Need, aWarning of Some Pitfalls, and Suggested Improvements in Its Application - Dendroclimatology - page 143

The rationale behind 'signal-free' standardization is that it should be possi-
ble to produce an improved (i.e., locally unbiased) chronology if the individual
measurement series could be detrended without allowing the fitting of standard-
ization curves to be affected by the presence of climatically forced variability.
One suggestion for achieving this condition is to remove the common variability
(chronology signal) from all measurement series to yield less-biased detrending
curves.

Implementing Signal-Free Standardization

Melvin and Briffa ( 2008 ) describe one such approach, applied in the context
of 'curve-fitting' standardization using options offered in the ARSTAN program
(Cook 1985 ) . They demonstrate how a combination of the 'segment length curse'
and localized distortion of standardization curves can produce a biased ring-width
chronology, apparent as a failure by the standardized chronology to express recent
climatic trends. Details of their implementation of this signal-free approach, in the
context of curve-fitting standardization, are given in Melvin and Briffa ( 2008 ) .
When this approach is applied in the case of regional curve standardization
(RCS), a first chronology is produced by division of the tree-ring measurements by
the appropriate RCS curve values. Each original measurement value is then divided
by the appropriate chronology value for that year to produce a first set of 'signal-
free' measurements. The standardization is then repeated on these signal-free data,
and a new chronology is produced from the new signal-free indices. The process is
repeated until the point where the signal-free data make up a chronology that has
virtually zero variance. In practice, the magnitude of residual bias (i.e., as repre-
sented by the variance of the signal-free chronology) after each iteration is 20% of
its initial value. A zero-variance signal-free chronology is considered here to be one
where all values are within the range 1.0 0.002. This condition is achieved gen-

erally within four or five signal-free iterations. At this point, the final RCS curve
is unaffected by any external growth-forcing signal and should, therefore, yield a
less-biased chronology.

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