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Instrumentation tends to change from one run to the next.
Filters get broken or lost, or deteriorate and are replaced.
Detectors change with age, may be destroyed by careless users, or just
die for unknown reasons.
Instruments suffer ``improvements'' that change their characteristics.
Only data taken with a fixed configuration can be reduced together.
Usually, this will include all of the data taken during a run; occasionally,
an equipment change is necessary during a run.
Data describing a fixed instrumental configuration can naturally be
grouped together in a table describing each particular run, or part of a run.