An investigator compared the antimicrobial effectiveness of two novel synthetic peptides, Peptide-X and Peptide-Y, against Staphylococcus aureus using the agar well diffusion method.
| Substance | Diameter of zone of inhibition (mm) |
|---|---|
| Peptide-X | 28 |
| Peptide-Y | 18 |
The investigator concluded that Peptide-X was more effective at inhibiting the growth of Staphylococcus aureus than Peptide-Y.
State two variables that must be controlled in this experiment to ensure this conclusion is valid.
