Any successful experiment requires careful design beforehand. There are systematic methods for ensuring the validity of test results.
To reduce variables in any type of experiment, it is recommended to include both positive and negative controls in the experimental design. Negative controls are particular samples included in the experiment that are treated the same as all the others but are not expected to change from any variable in the experiment. The positive control sample will show an expected result, helping the scientist understand that the experiment was performed properly.
We do not want standalone samples for proving the assumptions: in practice we would aim for increasing the number of samples to improve te statistical significance of our results.