Epidemiologic studies may be descriptive in nature (describing the frequency or characteristics of events) or analytic (testing relationships between common traits and outcomes).  Differing forms of descriptive studies exist.  These designs are outlined in the next slide.

Analytic studies include experimental designs (the randomized controlled trial) and observational designs (case-control studies, cohort studies, etc.).  The case-crossover study design has received a lot of attention in the past few years.