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Observational studies usually focus on the characteristics or distribution of phenomena in the population that you are investigating. Such studies may analyze data at one point in time or explore time trends in the relevant variables. They may be based on observations of individuals within a sample, or they may consider the relationship between variables observed in groups of subjects (for example, differences in diet and disease rate between countries). They are often the basis for hypothesis generating, rather than hypothesis testing.

Case studies are reports of potentially generalizable or particularly interesting phenomena. Individually, a case study cannot provide evidence that will help you to establish the truth of your hypothesis. Consistent findings across several case studies may provide support for an idea, but cannot be used in themselves to test a hypothesis.

Descriptive studies are careful analyses of the distribution of phenomena within or between groups, or a study of relationships existing between two or more variables within a sample. Descriptive studies are often well suited to qualitative examination of a problem (e.g. examining the coping strategies used by families on low income to ensure an adequate diet for their children when other demands [like the gas bill] are competing for limited cash). But of course they also provide descriptions of quantitative observations for single variables (e.g. how much money is spent on food, fuel, etc. in families on low income), or multiple variables (e.g. how money is spent on food in relation to total income or family size). Many epidemiological studies fall into this category (see below). They are useful for understanding the possible links between phenomena, but cannot in themselves demonstrate cause and effect.

Diagnostic studies establishing the extent of variation in disease states. They are helpful when selecting subjects for a study and deciding on which endpoints may be relevant when designing a study to explore cause and effect.