I am about to do a laboratory experiment in the scientific field of soil ecology and hydrology. Beforehand I want to make sure not to make any crucial mistakes, and therefore I would appreciate any hints and comments from your side. The main issue is how to deal with high natural variability (~25%) and a rather small sample size (total max=20).
Short Description of the Experiment:
We will put soil cores in cylinders and keep them under different moisture scenarios. Three different kind of Carbon forms will be measured for about one month.
We want to know if the variables change within groups and between groups.
The experimental design that was proposed is as follows:
There will be 2 different soil types, 2 different treatments + one control, and each treatment should be replicated three times. The total number of cylinders is thus 18 cylinders.
The variability in field measurements can be as high as 25% within one group. Due to pracitcal reasons there can not be more than 20 cylincers in total.
- Would it make more sense to have only one treatment and one control, but each one is replicated five times?
- Will I be able to draw any reliable conclusions from this kind of experiment under these conditions?
- How should I set the parameters to make some power calculations using e.g. G*Power 3? Which test should I choose? What should I set the effect size to and what should the numbers for df be?
- How should I analyze the data after the experiment is done? Should I use ANOVA? Can I use a mixed effect model?