I need urgent help with figuring out some statistical test for my biology experiment!

Question

So I tested the inhibitory effect of 2 essential oils on bacteria at 5 different concentrations increasing by 20% beginning with 10% and ending at 90%. Thus, the first independent variable is the type of oil used and the second is the concentrations. My dependent variable is the measure of the inhibition zone produced.

After that, I combined the oils in binary combination in 5 different ratios (like 10:90, 30:70, and so on) to see their inhibitory effect. If I am assuming correctly, that is a third independent variable.

Ultimately, I am trying to deduce whether the individual effect is better than the combined effect.

Initially, I thought that I could just run a one-way ANOVA followed by Tukey’s HSD test for the combined effect results and then run a two-way ANOVA with replication (since i had 10 trials per conc.) followed by Tukey’s HSD test. However, I then realized I can’t do Tukey’s HSD test for the two-way ANOVA since I only have 2 groups (2 extracts) when it has meant to be for 3 or more. Is that true?

Now, I am unsure of which statistical test to run in order to determine whether the individual effect is greater than the combined. Which tests exactly should I proceed with? Do I still carry on with the one-way for the combined effect and if so what would I do for the individual effect in order to see which effect is greater at inhibiting bacteria than the other?

Answer 1

If I am understanding your experiment correctly, you took 2 different essential oils and are using 5 different concentrations of each to determine the inhibition zone produced by each concentration and performed 10 trials per concentration to suffice the repetition requirement.

If that is the case, doesn't that mean you have 10 groups? 5 groups (ie concentrations) per essential oil, which would have their own inhibitory zone averages. If this is correct, then your test requirements should be fulfilled. This would be the case if you are specifying essential oil + conc. in your final analysis, rather than just essential oil.

But if you are creating just a trend and using it to broadly answer whether one essential oil is more effective than the other vs combined then it would only be two groups, but I suppose my confusion lies with what your hypothesis is and how you want to create your final conclusion?