What Are Some Common Mistakes?

Here are some of the things to look out for:

Choosing Hypotheses that are difficult to Test
Pick a hypothesis that may be tested easily. Consider time and resources. Most of us do not have access to fancy equipment. Make sure you can perform your experiment with common household supplies and materials, and with enough time for completion.

Testing Multiple Variables
Test only one variable. If you change more than one condition in the experiment, you won’t know which of the conditions produced the results you observed. 

Taking Poor Notes
Take good notes as you make your observations, so you get useable data. A good scientist includes all observations, even the ones that go against the original hypothesis. Many scientific discoveries have been made by looking at the data that did not “fit” what was thought to happen.

Creating Unclear Displays
A poor presentation can make even a good experiment look bad. Your display should be easy to understand, be neat and attractive, and have good illustrations, including photographs of the different steps in the experiment.