sciencepalooza! – Experimenting
Designing Your Hypothesis and Variables
There is a difference between messing around and science. In messing around you mix a little of this with a little of that to “see what happens.” This is not only potentially dangerous, but also a waste of time. In science we do background research to find out as much as possible about what has been done and what we EXPECT to happen—based on the thousands of years of scientific evidence and reasoning we have accumulated.
One definition of a hypothesis is “an educated guess.” When you conduct an experiment, you’re not guessing what will happen, instead, you’re predicting what you’re pretty sure is going to happen based on the science you’ve already done or read. While you may read that a hypothesis is written as an “if-then” statement, a well-written hypothesis should really consist of three parts:
because (some science you know)
if (this is done)
then (this will happen).
Here’s an example of a well-written hypothesis:
Because triclosan is toxic and builds up in the environment, if it is added to water in which brine shrimp live then the number of brine shrimp will decrease over a one-month period.
If you write a well-designed hypothesis, then your variables will already be part of it. The independent variable is the cause or the “if” clause of the hypothesis. For example, in our antibacterial soap experiment, the independent variable is the concentration of triclosan. This is the factor you will change. The independent variable should be tested by ranges or degrees, rather than by all or nothing. For experiments with living things, temperatures should be tested, starting from normal, in 5-degree variations. However, when chemicals are tested we usually use a serial dilution so that concentration changes by a factor of 10 (10%, 1 %, 0.1%, .001%).
The dependent variable is the effect (what you expect to happen), or the “then” part of the statement. It is the result you will measure. In our antibacterial soap experiment, the dependent variable is the number of brine shrimp alive after each week for 4 weeks.
It’s easy to get the term “controlled variables” confused with “control group.” The controlled variables are every factor you keep the same between the test groups. e.g. amount of water, temperature, and light exposure. A control group is an experiment conducted without using the independent variable.
The control group would be a tank of brine shrimp exposed to the same water, light, temperature, and food, but without triclosan