I really like the inquiry "What variables influence outcomes" (p.114) and the example "a fender bender car accident" (p.115) that Creswell presents to explain the definitions of dependent, independent, control, and confounding variables. It is easier to understand than those theoretical explanations and gives me a vivid picture of what the family of variables looks like.
In my understanding, the most important thing to consider when studying the relationship between variables (i.e. independent and dependent variables) is that the relationship does exist or has the high potential to be established, or your research is only a fiction.
Differences between quan and qual
The cause-and-effect logic in quantitative research is more like the thinking pattern in natural science; while the exploration of a central phenomenon from multiple aspects in qualitative research is more like what the social scientists usually do.
In quantitative research, the researcher imposes his/her own view in the research site (i.e., researchers choose certain variables to study) and aims to develop a consensus of opinion from all participants. And the audience of research learns from what the researcher seeks to know. In qualitative research, the researcher usually begins with a general idea to explore, then develops or emerges research questions according to the participants' view during the process of both data collection and data analysis.
Work consulted:
Creswell, J. W. (2012). Chapter 4 in Educational Research: Planning, Conducting and Evaluating Quantitative and Qualitative Research. Pearson Education: Boston, MA.