On Fri, 9 Sep 2022 21:46:41 +0800, Jinsong Zhao <
jsz...@yeah.net>
wrote:
>Hi there,
>
>We have collect some data from an ecological experiment.
I will start out by saying that this sort of 'ecological experiment'
is alien to me, including the vocabulary describing it. I had to
read this four times before it I began to translate it to my own
terms. I think that others reading this group will have the same
trouble. I offer my 'translation' to concrete terms, but the actual
details, if you provide them, might remind others of problems
that they have done or worried about.
>
>For each treatment, 4 out of 7 species are chosen, and incubated in one
>unit. And then some properties, e.g., the sum of individual weights in
>the unit, are measured. Totally, 35 treatments are established. For a
>specific property, it varies among different treatments. It makes sense
>because of the interaction between species.
Okay. An experimental unit (as I translate) is a Petri dish. Taking
4-of-7 for each dish results in a complete set of 35 dishes, one for
each unique combination. I speculate that you should do this at
least twice, so that you have Within variance to use as Error.
>
>To my knowledge, I could design a 2^7 factorial experiment to test the
>main and interaction effect of the 7 species. This is a huge experiment
>and may not be feasible. However, I don't know the design of such
>experiment design or how to analyze those data.
You can lay out a 7-way ANOVA. It certainly will be missing the
5-way, 6-way, and 7-way interactions that are absent in the data.
Testing for main effects and simple 2-way interactions is easy,
with 28 total DF. I think you need that replication to get a more
robust error term for the interaction tests. There will be 35 df
for the 3-way interactions (if I'm not screwing up); so testing
that-plus-28 DEFINITELY needs an N more than 35, and it
will be better testing with N of 70 or more.
Doing so many tests as even the 2-way interactions, without
guidance of a-priori hypotheses, certainly puts you in the position
of 'too many tests' -- the multiplicity problem. Someone might
be concerned even about the 7 main effects as being 'many'.
I have no idea how BIG you expect your effects to be. Or how
big they need to be in order to matter. So I can't say whether
you should replicate the 35-dish experiment twice or five times.
Even 10 times might be a good idea, if you are fishing among a
number of 'properties' as equal aternatives for outcome.
Hope this helps.
--
Rich Ulrich