One question about expriment design, please help!

#1
I am a little confused about nested design and block design. Please see following example, how do you think? It is a block design or nested?

Six groups of oysters (3 male groups and 3 femal groups) were selected from a clean site and six groups of oysters (3 male groups and 3 femal groups) were selected from a polluted site in a tidal creek.(Oyster size is a reasonable proxy for gender)

Six oysters from each group were sacrificed and morphological measurements were made - the response was an average morphological measurement from each group of a half-dozen oysters. Afterwards, the six group of oysters from the clean site were transferred to the polluted site and the six groups of oysters from the polluted site were transfered to the clean site. After six weeks, six more oysters from each group were sacrificed and the measurements were repeated.


1) can you identify the design?

At first I think it is a nested and crossed design, since groups are nested in gender and sites are the crossed factor. But later, I doubt it. Since I think if I use block design I still can explan it. Block design means a set of units that are homogeneous in some sense. In this example, I think block design still makes sense.

2) Is there any serious confounding in the experiment?
I think this experiment has both fixed and random factors. So we should consider which model to use restricted or unrestricted.

3) Can both gender and treatment (clean/polluted) effects be tested?
I think both effects can be tested.

Any advices would be appreciated! Thanks!
 
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