The handbook consists of a solid theoretical and scientific rationale that is presented in a simple language, which both the beginning and advanced students can understand. It also presents a balance between quantitative and qualitative methods of research and analysis, and advocates for problem-focused methodology and mixed design when the questions asked by the researcher or the scientists require doing so. The most distinctive feature of the book is that the contents are presented in a hierarchy in terms of complexity. Therefore, the handbook can be used for teaching simple topics such as asking questions that deserve scientific methods of investigation, and simple statistical techniques, as well as complex multivariate methods of inquiry. The mathematical terms are presented in symbols and graphs only when the concepts were clarified in a simple language and friendly manner. Each of the chapters develops in a clear and sequential order, so that students and researchers accumulate knowledge based on concept mapping rather than memorization. The didactics of the book enable the learner to carry over the learning contents to other courses and apply them to other domains of interest.