Probably the animals and what the animals live in.
The answer is <span>Take consistent amounts of saliva samples from a number of people who have not eaten in 8 hours and measure the quantity of amylase in each sample.
First, the researcher needs to know the quantity of amylase in the <u>same</u> amount of saliva. Thus the amount must be consistent and not varying.
Next, people must not eat in 8 hours, so they are all in the same condition at the moment of collecting saliva. If some eat immediately before the sample collecting while some eat 8 hours before, the content of their saliva will be different.
For the correct results, all conditions must be the same.</span>
