The document discusses different types of non-experimental quantitative research designs, including descriptive studies, relationship studies, comparative studies, correlational studies, prediction studies, causal-comparative/ex-post facto studies, and surveys. Relationship studies are important in non-experimental designs because they allow researchers to identify possible causes, variables to study, and ways to predict the value of one variable based on others. The key difference between cross-sectional and longitudinal surveys is that cross-sectional surveys collect information from samples at one time, while longitudinal surveys study the same group of subjects over an extended period of time.