Final answer:
The error described is that the students changed more than one variable, which can lead to skewed results that don’t accurately reflect the effect of the single intended treatment.
Step-by-step explanation:
The statement that best describes an error the students made in this investigation is: The students changed more than one variable. A fundamental principle in experimental design is that only one variable should be manipulated at a time in an experimental group, while all other variables are held constant. This allows the researcher to attribute any differences in the outcome directly to the manipulation of the single variable. The use of a control group is essential as well, where this group does not receive the experimental treatment and serves as a basis for comparison.
For instance, if students are investigating the effect of wind on evaporation and they use both natural wind and an electric fan, their results could be skewed because the wind speeds can differ significantly; this would be changing more than one variable (type and speed of wind). Such an error could invalidate the experiment's findings because it would be unclear whether any differences observed were due to the type of wind or other confounding factors.
Controlling variables like the amount of water, the type of soil, etc., are crucial for drawing accurate conclusions about the impact of the experimental treatment alone. The lack of a proper controlled experiment could lead to incorrect interpretations of the data.