It is important to note that when a model component is established,
if a dataset has been input, then estimates based on the
(filtered) data are automatically made for the initial parameter values
and their minimum and maximum ranges.
However, if no dataset has been input,
then the initial parameter values and ranges are assigned arbitrary
default values.
If multiple datasets have been input, and if no dataset is indicated in
the model expression, then
automatic parameter estimates are based on the lowest numbered dataset;
for instance, if the user inputs
sherpa> DATA 1 example1.dat
sherpa> DATA 2 example2.dat
sherpa> POLY[modela]
then the estimates of modela's parameter values are based on the first
dataset.
Otherwise,
if multiple datasets have been input, and a dataset is indicated in
the model expression, then
the automatic parameter estimates are made based on the
indicated dataset; for instance, if the user inputs
sherpa> DATA 1 example1.dat
sherpa> DATA 2 example2.dat
sherpa> SOURCE 2 = POLY[modela]
then the estimates of modela's parameter values are based on the second
dataset.
The user may always obtain updated estimates using the
command GUESS.
Updated estimates may be needed if:
-
a dataset is input,
-
or an instrument model is specified,
-
or a dataset filter is changed,
-
or a dataset COORD
analysis setting is changed,
after a model expression is input.
The user may of course wish to directly set
initial parameter values and ranges;
see the CREATE command.
Establish a model component after data input:
sherpa> ERASE ALL
sherpa> DATA example.pha
...
sherpa> POW[modelc]
modelc.gamma parameter value [1]
modelc.ref parameter value [1]
modelc.ampl parameter value [1.08261e-05]
sherpa> SHOW modelc
powlaw1d[modelc] (integrate: on)
Param Type Value Min Max Units
----- ---- ----- --- --- -----
1 gamma thawed 1 -10 10
2 ref frozen 1 2.7614e-02 14.5494
3 ampl thawed 1.0826e-05 1.0826e-07 1.0826e-03
In this example, the ERASE ALL
command is used
to remove the previous model setup. Next, data
are input, with the command DATA example.pha.
This is then followed by the establishment of
the model component, with the command POW[modelc].
Notice that the given initial model parameter values
and ranges are estimates based on the filtered data.
Establish a model component prior to data input;
obtain parameter estimates using GUESS:
sherpa> ERASE ALL
sherpa> POW[modelc]
modelc.gamma parameter value [1]
modelc.ref parameter value [1]
modelc.ampl parameter value [1]
sherpa> DATA example.pha
...
sherpa> SHOW modelc
powlaw1d[modelc] (integrate: on)
Param Type Value Min Max Units
----- ---- ----- --- --- -----
1 gamma thawed 1 -10 10
2 ref frozen 1 -3.403e+38 3.4028e+38
3 ampl thawed 1 0 3.4028e+38
sherpa> GUESS modelc
sherpa> SHOW modelc
powlaw1d[modelc] (integrate: on)
Param Type Value Min Max Units
----- ---- ----- --- --- -----
1 gamma thawed 1 -10 10
2 ref frozen 1 2.7614e-02 14.5494
3 ampl thawed 1.0826e-05 1.0826e-07 1.0826e-03
In this example, the
ERASE ALL command is used
to remove the previous input data and model component setups.
Next, a model component is established prior to input of data
(with the command POW[modelc]), and
so the given initial parameter values are default values. The
GUESS command is then used to obtain
parameter value and range estimates, based on the input
dataset.
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