Type I and Type II Errors 

 A well-known social scientist once confessed to me that, after decades of doing social research, he still couldn't remember the difference between Type I and Type II errors. Since I suspect that many others also share this problem, I thought I would share a mnemonic I learned from a statistics professor. Recall that a Type I error occurs when the null hypothesis is rejected when it is in fact true, while a Type II error occurs when a null hypothesis is not rejected when it is actually false. This distinction, of course, many people find difficult to remember. 

 So here's the mnemonic: first, a Type I error can be viewed as a "false alarm" while a Type II error as a "missed detection"; second, note that the phrase "false alarm" has fewer letters than "missed detection," and analogously the numeral 1 (for Type I error) is smaller than 2 (for Type I error). Since learning this mnemonic, I have not forgotten the difference between Type I and Type II errors!