Difference between replication and repeated measurements


from Montgomery’s Experimental Design:

There is an important distinction between replication and repeated
measurements
.

For example, suppose that a silicon wafer is etched in a single-wafer
plasma etching process, and a critical dimension on this wafer is
measured three times. These measurements are not replicates; they are
a form of repeated measurements, and in this case, the observed
variability in the three repeated measurements is a direct reflection
of the inherent variability in the measurement system or gauge.

As another illustration, suppose that as part of an experiment in
semiconductor manufacturing, four wafers are processed simultaneously
in an oxidation furnace at a particular gas flow rate and time and
then a measurement is taken on the oxide thickness of each wafer.
Once again, the measurement on the four wafers are not replicates but repeated measurements. In this case they reflect differences
among the wafers and other sources of variability within that
particular furnace run.

Replication reflects sources of variability both between runs and (potentially) within runs.

  1. I don’t quite understand the difference between replication and
    repeated measurements. Wikipedia says

    The repeated measures design (also known as a within-subjects design) uses the same subjects with every condition of the research,
    including the control.

    According to Wikipedia, the two examples are in Montgomery’s book
    aren’t repeated measurement experiments.

    • In the first example, the wafer is used under only one condition, isn’t it?

    • In the second example, each wafer is used with only one condition: “processed simultaneously in an oxidation furnace at a
      particular gas flow rate and time”, is it?

  2. “Replication reflects sources of variability both between runs and
    (potentially) within runs”. Then what is for repeated measurements?

Thanks and regards!

One Response to “Difference between replication and repeated measurements”

  1. Peter Flom says:

    I don’t think his second example is replication OR repeated measurements.

    Any study involves multiple cases (subjects, people, silicon chips, whatever).

    Repeated measures involves measuring the same cases multiple times. So, if you measured the chips, then did something to them, then measured them again, etc it would be repeated measures.

    Replication involves running the same study on different subjects but identical conditions. So, if you did the study on n chips, then did it again on another n chips that would be replication.

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