A-Level biology required practical 6 - Aseptic techniques

Hi, tomorrow I am doing the practical where we use aseptic techniques to investigate the effect of antimicrobial substances on microbial growth. I just have a few questions about the variables:
- what would the independent and the dependent variables be?
- what are the control variables and what strategies could minimise their impact?
Thanks!

You are going to be changing the antiseptic (either concentration, or type of antiseptic).
You are measuring the zone of inhibition, on the agar petri dish.
You ensure you add the same volume of antiseptic, you incubate the agar with bacteria evenly. Your negative control would be something that is not antiseptic (such as water), to prove that the agar or paper dish want not killing the bacteria.
Petri dish store upside down (condensation does not fall onto agar), lightly tapped on lid (allow oxygen in).