Question

A scientist wants to test whether the pH, salinity, and temperature of seawater impacts coral development. He sets up an experiment with three conditions: Aquarium 1: Normal pH, normal salinity, normal temperature Aquarium 2: High pH, high salinity, high temperature Aquarium 3: Low pH, low salinity, low temperature He finds that coral grow the least in Aquarium 2. He concludes that coral does not grow well in conditions with high salt. Why is this an invalid conclusion?
 The experiment does not have a control group.
 The experiment changes more than one variable at once.
 The experiment does not have an experimental group.
 The experiment does not measure any dependent variables.

Answer 1

The conclusion that coral does not grow well in conditions with high salt is invalid because (B)  The experiment changes more than one variable at once.

In order to correctly study the effect of three variables on there needs to be, 3 x 3 x 3 = 27 different aquariums (every variable takes 3 different values and there are 3 different variables).

In these aquariums, only one condition needs to be varied at a time to study its effect. To study the condition with high salt, the temperature and pH needs needs to be kept fixed at a particular value whilst varying the salinity from normal to low to high.