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# Algebra level symbolic math

• ### Probability Sampling

This site describes in detail 5 different types of random sampling, giving examples, definitions, and procedures.
• ### Nonprobability Sampling

This site describes numerous methods of nonprobability sampling, including accidental, haphazard or convenience sampling and the many types of purposive methods.

• ### Factorial Design

This site explains the 2x2 factorial experimental design, it's components, and it's effects. Graphs illustrate the concepts discussed.
• ### HyperStat Online: Ch. 4 Introduction to Probability

This chapter of the HyperStat Online Textbook presents six sections on probability including conditional probability, unions, intersections, and the Binomial distribution. A link to exercises for the chapter is available, as well as a link to an applet that demonstrates the approximation of the binomial with a normal distribution (in section 5).
• ### Z-test for difference between means

A slideshow presentation with two good examples on using the Z-test for the difference between two means. Gives some good "plain language" interepretations of what "statistically significant difference" means.
• ### Research Methods Knowledge Base : The t-Test

This particular textbook lesson addresses the independent t-test. It presents to the user how to compute the t statistic and then how to interpret the results.
• ### Student's t-test Applet

This page gives a short background on Student's t-test and provides three t-test calculators. Two perform t-tests for independent groups and one performs t-tests for matched pairs. Users type in individual data points or copy and paste the entire data set. Some examples are given for demonstration.
• ### Two-Sample t-Test for Equal Means (Engineering Statistics Handbook)

Gives textbook-like explanation with some real-life data to compute a t-test and determine if two population means are equal. Also has some links for case studies and a web-based program called Dataplot. There is a printer-friendly version on the main homepage (see source).
• ### Comparing Two Independent Sample Means

Textbook-like example showing the independent t-test. Gives a nice way for students to think through the problem and interpret results.
• ### Statistics at Square One: The t tests

Gives very detailed explanation of t-tests (confidence intervals, one-sample, two sample independent, two sample paired, pooled and unpooled variances). Discusses the assumptions that are made for each type of t-test. This topic is part of an online textbook.