1. A researcher is looking at whether playing violent video games versus non-violent video games affects aggressive behavior
2. A researcher is examining whether students improve after taking a memory skills training class in comparison to before.
3. A researcher is looking at happiness level of different groups of couples. She divided them into those who has great communication skills and bad communication skills
4. A researcher records daily caffeine consumption and anxiety level of a group of college students. The researcher would like to determine whether there is a significant relationship between caffeine level and anxiety.