The calibration model is a linear mixed-effects model (LMM) that fits the
isotopic values of sedentary organisms as a linear function of the isotopic
values in their environment (e.g. precipitation).
This function considers that the isotopic values from the environment (e.g.
from precipitation) at the locations at which organisms were sampled are not
known. The function therefore predicts these isotopic values from the
geostatistical model fitted by the function isofit
, which is
provided to calibfit
using the argument isofit
.
The LMM used to fit the calibration function has a simple fixed-effect
structure: an intercept and a slope. The random effect is more complex: it is
normally distributed with mean zero, a certain variance between locations
proportional to the squared fixed slope, and a covariance structure defined
by the prediction covariance matrix of the isoscape model between the
calibration locations. All models used in IsoriX will be soon detailed
in an additional document.
This function is only needed in the case for which the assignment of
organisms has to be performed within an isoscape that was built using another
source of isotopic values (e.g., precipitation). This implies that if the
isoscape had been fitted using isotopic ratios from sedentary animals, then
this calibration step is not needed.
If source isotopic values are known at the locations where sedentary
organisms were sampled, users should calibrate their data directly using the
function lm
by fitting tissue isotopic values as a
function of source isotopic values.