Science lab graph; independent/dependent variables

Question

So, uh...

In biology we did this lab where we recorded--for six minutes--the movements of pill bugs from one petri dish to another; (there were two connected together, one damp and one dry) it was basically this: http://bioweb.wku.edu/courses/Biol114/Behavior/PIll_bug1.asp

And so now I have to do a graph recording my data. But, like... I can't decide what the independent/dependent variables are. I'm assuming that the two options I have are "time" and "movement of pill bugs from dry to damp" or something along those lines. I just don't know which is supposed to be which.

Also, what would be better for this? A line or bar graph?

Answer 1

If you do the little demo lab on the webpage, it will give you a data table with the following columns:  time, # in wet chamber, # in dry chamber.

Clearly, the DEpendant variable here is the number of bugs in each chamber.  What might be confusing is that it seems like this would depend on dampness.  But think about what you measured the number against ... you measured against time.  So the variable that the number of bugs depended on was time.  If you had a different experiment where you had different levels of dampness, that might be a different story (in that case, you could plot bugs vs dampness).

Note you have two different dependant variables -- bugs in the dry room, and bugs in the damp room.  So your graph should contain two plots.  You could use either a barplot or line plot -- personally I find bar graphs that contain more than one dependant variable hard to follow, especially if they cross.  Also, bar graphs use a lot of ink to convey very little information -- so in general a scatter plot (or a line plot if you will) is better.  

To avoid confusion with lineplots, you should be sure to indicate the data points where you actually collected data with small dots, circles, squares, etc.  That way, you make it clear that data was recorded every 30 seconds, and wasn't somehow recorded continuously.