This website is a collection of analysis tools commonly used in statistics and mathematics. These tools are divided into 7 categories: 1) Summarizing Data 2)Computational Probability 3)Requirements for most tests and computations 4) One population and one variable 5)One population and two or more variables 6)Two or three populations and one variable 7) Several populations and one or more variables
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.