Science Buddies: "Ask an Expert"

Hi,
My son has done science fair project on lizard brain lateralized predation. He put crickets in his lizards cage and observed whether they used their right or left eye to watch/attack prey. His dependent variable is the number of right and left attacks. He/we are not sure of the independent variable. From what I have read he has done an observational study (not and experiment). Observational studies do not have independent variables. As was stated in another location here in the forum, his teacher has been well aware are his project. But the fair instructions call for him to list his DV and IV. For independent variable, should he state something like "none, observational study"? Or is his putting the prey into the cage his independent variable? Certainly, on a graph, his x and y variables will be left/right attacks vs number of prey eaten.
Thanks
Confounded

An independent variable would be anything manipulated -- did he feed different prey at different times or in different places in the tank, to see if that affected which eye was used to hunt?

Otherwise, I believe you are correct, there would be no independent variable. (but, I'm not an official "expert" here).

You should be able to list lots of controlled variables -- same tank, same lizard, etc.

Hi Confounded,

You have an excellent question! It is true that the independent variable is typically the one that you (as the scientist) have manipulated, but you can still define an independent variable for an observational experiment. The independent variable is the one upon which the dependent variable "depends." In other words, what might be some thing(s) that would make the lizards' right/left preference change?

The simplest independent variable in your son's experiment is individual. Assuming your son observed more than one lizard, each individual may have a different frequency of right/left attacks. So he can consider the individual lizard to be the independent variable, since the preference may depend on the individual.

You can also look a posteriori (after the fact) for other factors. If these lizards are easy to sex, you could see if there are differences between males and females. Size (mass, or length) may also be a factor. If you see differences based on any of these factors, they would count as independent variables.

I hope this helps. Please post again if you have any more questions.

Heather