This could, for example, be mail first and
then follow up with telephone and face-to- face interview.
The US Census Bureau has that in place.
The pros, you maximize response rate, and you reduce the cost.
You can maximize quality and efficiency.
The cons,
well, you might have mode effects.
People react to the mode
of administration, oral or visual presentation,
in different ways, and, likewise, the response option might have an effect.
Also different people are recruited by different modes,
they answer, as I said, differently by the different modes,
and these differences in the answer reflect true
differences, the composition or response errors, you don't know.
So it's good to look at many of these mixed modes
that is out there now, to see what can you build
into your design so that you can at least measure some
of these effects or disentangle them as you have the data.
Here's another question that was posted online.
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And the general answer to these types of questions is,
you should always try to maximize quality in any given mode.