Answer: Some possible bias in an experiment about how sleep affects mood could be a person's average mood with or without sleep, how people are acting in the experiment compared to how they actually would in their normal environment, or even how the person collected the data for the experiment interprets their moods.
Step-by-step explanation:
1. A person's mood could be relatively different depending on other factors than just sleep, so it could affect experiment. For example if an emotional crisis had occurred recently, it could affect your mood and environmental factors could affected your mood, like weather.
2. A person could also change their mood around strangers or even when participating in the experiment. For instance if they aren't comfortable with the person taking down the analysist or the other test subjects, they make act differently than they usually do at home where they're comfortable.
3. A person collecting the data will have to know how to specify and determine a persons mood with their instruments. For example, if they're doing it just by talking with the test subjects, they'll have to know how to properly understand their different stages of each of their moods with out the data collectors mood changing as well. Also, if the data collectors mood is changed at all it can slightly affect the experiments results as well.