This presentation on data analysis addresses observational studies and randomized controlled trials in two different sections. Types of studies are defined and examples of each study is given to emphasize the differences. Factors and variables are also discussed.
Use the Sample Size Calculator to determine the sample size you need in order to get results that reflect the target population as precisely as needed. You can also find the level of precision you have in an existing sample. The site also describes terms you need to know to understand confidence intervals and what they mean.
This resource defines and explains standard deviation and the normal distribution.