Confounding variables

Question 1

what can affect the dependant variable?

Answer 1

Due to the complexities of biological systems, other variables besides the
independent variable may affect the dependent variable

Question 2

what must be held constant?

Answer 2

These confounding variables must be held constant if possible, or at least
monitored so that their effect on the results can be accounted for in the
analysis

Question 3

randomised block design

Answer 3

In cases where confounding variables cannot easily be controlled, a
randomised block design could be used

Question 4

how can they be distributed?

Answer 4

Randomised blocks of treatment and control groups can be distributed in
such a way that the influence of any confounding variable is likely to be
the same across the treatment and control groups.