Statistics How To

Statistics Definitions > What is the Tukey Ladder of Powers? The Tukey ladder of powers (sometimes called the Bulging Rule) is a way to change the shape of a skewed distribution so that it becomes normal or nearly-normal. It can also help to reduce error variability (heteroscedasticity). Tukey (1977) created a table of powers (numbers … Read more

Statistics Definitions > Proportion of variance What is Proportion of Variance? “Proportion of variance” is a generic term to mean a part of variance as a whole. For example, the total variance in any system is 100%, but there might be many different causes for the total variance — each of which have their own … Read more

Probability > Base Rates and the Base Rate Fallacy. What are Base Rates? The term “base rates” has a slightly different meaning depending on where you use it. In general, a base rate is the probability of some event happening. For example, your odds of being struck by lightning in your lifetime is currently about … Read more

Pearson Correlation Coefficient > Restricted Range What is a Restricted Range? A restricted range is a range of values that has been condensed, or shortened. For example, the entire range of G.P.A. scores is 0 to 4.0. A restricted range could be 3.0 to 4.0, or 2.0 to 3.0. Restricted Range and Correlation Restricted ranges … Read more

Statistics Definitions > What is the Likert Scale? A Likert Scale is a type of rating scale used to measure attitudes or opinions. With this scale, respondents are asked to rate items on a level of agreement. For example: Strongly agree Agree Neutral Disagree Strongly disagree Five to seven items are usually used in the … Read more

Statistics Definitions > Shape Parameter What is a Shape Parameter? A shape parameter, as the name suggests, affects the general shape of a distribution; they are a family of distributions with different shapes. The parameters are usually known from prior statistical data or they are sometimes estimated from current data. For example, the Beta distribution … Read more

Design of Experiments > Survey Sampling Contents: Survey Sampling Survey Research What is Survey Sampling? Survey sampling is selecting members from a target population to be in a sample for a sample survey. Usually the survey is some type of questionnaire (i.e. in-person, phone or internet survey). The science of survey sampling has greatly transformed … Read more

Design of Experiments > Random Selection and Assignment What is Random Selection? Random selection means to create your study sample randomly, by chance. Random selection results in a representative sample; you can make generalizations and predictions about a population’s behavior based on your sample as long as you have used a probability sampling method. The … Read more

Descriptive Statistics > Measures of Variation Variation is a way to show how data is dispersed, or spread out. Several measures of variation are used in statistics. Different Measures of Variation The Range A range is one of the most basic measures of variation. It is the difference between the smallest data item in the … Read more

Statistics Definitions > Item Response Theory What is Item Response Theory? Item response Theory(IRT) is a way to analyze responses to tests or questionnaires with the goal of improving measurement accuracy and reliability. If you want your test to actually measure what it is supposed to measure (i.e. mathematical ability, a student’s reading ability or … Read more