Statistics How To

Statistics Definitions > Variance Sum Law What is the Variance Sum Law? The Variance Sum Law determines the variance of a sum (or difference) when you know the variance of the component parts. For example, suppose you ran a research project which involved sampling the weight of apples in New York orchards, and then you … Read more

Statistics Definitions > What is the Turning Point Test? The turning point test is a test which lets us check whether a set of random variables are independent and identically distributed. Proposed by Irénée-Jules Bienaymé in 1874, this test is still an important part of the statistical toolbox. In scientific applications, it is most commonly … Read more

Outliers > What is Peirce’s Criterion? Peirce’s Criterion is a method, proposed by Benjamin Peerce in 1852, that allows us to eliminate outliers from data sets. An outlier is a part of the data set that is abnormal and not representative of the general trend. Finding and eliminating outliers is an important part of much … Read more

Statistics Definitions > What is Stratification? Stratification means to sort data/people/objects into distinct groups or layers. For example, you might sort “All people in the USA” into ethnic groups, income level groups, or geographic groups. Fields that use this definition for stratification include the social sciences, where people are often sorted into groups by rank, … Read more

Types of Bias > What is Performance Bias? Performance bias happens when one group of subjects in an experiment (for example, a control group or an treatment group) gets more attention from investigators than another group. The difference in care levels result in systematic differences between groups, making it difficult or impossible to conclude that … Read more

Design of Experiments > What is a Latin Square? A Latin square is an ancient puzzle where you try to figure out how many ways Latin letters can be arranged in a set number of rows and columns (a matrix); each symbol appears only once in each row and column. It’s called a Latin square … Read more

Design of Experiments What is a Plackett-Burman Design? A Plackett-Burman design (a type of screening design) helps you to find out which factors in an experiment are important. This design screens out unimportant factors (noise), which means that you avoid collecting large amounts of data on relatively unimportant factors. For example, dozens of factors contribute … Read more

Design of Experiments > What is a Definitive Screening Design? A Definitive Screening Design (DSD) allows you to study the effects of a large number of factors* in a relatively small experiment. In simple terms, DSDs are an improvement on standard screening designs (like the Plackett-Burman) that prevent confounding of factors and can also detect … Read more

Bias in Statistics > What is Attrition Bias? Attrition bias happens when participants drop out from a study; The drop-outs have unique study-related characteristics, resulting in a difference between initial and ending samples. Selective attrition bias happens when the differences are between control groups and treatment groups (as opposed to beginning and ending samples). If … Read more

Design of Experiments > You may want to read about factors and blocks first. See: What is a Factorial Design? What is a Randomized Block Design? What is a Split-Plot Design? When some factors (independent variables) are difficult or impossible to change in your experiment, a completely randomized design isn’t possible. The result is a … Read more