Expert: Dana Krempels, Ph.D. - 10/7/2010

Question
Hello Dr. Krempels.
I noticed your response to a student who is doing her science fair project on who's mouth is dirtier dog, human or cat.  Well, as it turns out, I'm doing the same project and would like your guidance to make it awesome.  My hypothesis is "If humans, cats and dogs are tested for bacteria content, then the bacteria will decrease in the order from humans to cats to dogs."   Any assistance you're able to provide whether it be procedural or methodological would be greatly appreciated.  When I read your response regarding this topic, I must admit, I panicked because my goal is to make it to the State level.

Answer
Dear Carly,

What you have stated as a hypothesis is actually a prediction, not a hypothesis.

I'd start off by asking:  WHY do you think there is a "hierarchy" of bacterial concentration from human to cat to dog?  When you put forth a hypothesis, it must be from a position of strength and knowledge about the area, not just a wild guess.  At the moment, I suspect your prediction (not a hypothesis) is a wild guess, since the project you're describing is more of a pilot study.  Once you actually have results, the next question you might ask is   "WHY are there differences in the bacterial populations in the mouths of humans, dogs, and cats?" or something like that.  It's like climbing a tree, once you get started.  :)

Also consider:  how are you going to test your hypothesis?  For the particular problem you've set up, you will need a method by which you can *quantify* your results and then test them statistically.  And if you are using statistics (if you're comparing more than two species, you'll have to use Analysis of Variance (ANOVA); if you use two, then you'll use a t-test.  I would suggest the latter), then you will need to use statistical hypotheses:  null and alternative.

A sample null and alternative hypotheses for the question you are asking might be:

H0 (null) - There is no difference in the bacterial populations in the mouths of humans, dogs, and cats.

HA (alternative) - There is a difference in the bacterial populations in the mouths of humans, dogs, and cats.

You might *predict* that one species or another has a denser population of bacteria (how will you measure this?), but if you predict that, you will have to give logical reasons for that prediction.  (My suspicion is that there will be different *species* of bacteria in the mouths of these three mammals, but it will be very difficult for you to quantify how many are in each mouth.  You also have to consider the surface areas of the mouths, etc.  It can get very complicated very quickly.)

Also, you might want to avoid using a qualitative word like "dirtier."  Some bacteria are beneficial, and shouldn't be considered a problem.  Others are potential pathogens, and others might just be commensal species "along for the ride."  So remain dispassionate with your hypothesis and make conclusions about what this means in terms of hygiene only after you have analyzed your results.  (Though you could mention your *reason* for doing this study in your introduction, if it concerns public health.  And I can tell you now from looking at your prediction:  you're in for a surprise.)

If you have never used this type of paired hypotheses, then I will direct you here:

http://www.bio.miami.edu/dana/dox/scientific_method.html

which gives an overview of how hypotheses are formulated, and the different methods by which they are set up and tested.  I hope this will help get you started.  The most important part of a good scientific inquiry is the design, so that's where you should begin.

Hope this helps!

Dana