Skewness – Floor & Ceiling Effects

In this video I explain how skewness in a sample may be caused by a floor or a ceiling effect, in which the measurement isn’t able to capture variability at the low or high end of a population’s distribution. A floor effect will result in a positively skewed sample, while a ceiling effect will result in a negatively skewed sample, … Read More

Skewness – Positive & Negative Skew

In this video I explain one way in which a distribution can deviate from normality, which is skewness. I explain the difference between positive and negative skew, and how these can be seen in histograms, stem and leaf displays, and box and whisker plots. I then discuss the effect of skew on measures of central tendency and consider possible reasons … Read More

Exploratory Data Analysis: Box & Whisker Plots

In this video I describe box-and-whisker plots, or boxplots, developed by John Tukey. First we’ll look at a simple box-and-whisker plot, then we’ll see some variations depending on the data, some different sets of terminology that can be used when creating boxplots, and how we can use boxplots to get a quick visual sense of differences between distributions. For more … Read More

Exploratory Data Analysis: Stem & Leaf Displays

In this video I explain stem-and-leaf displays or stemplots, which are part of exploratory data analysis developed by John W. Tukey. I explain the basic design of stem-and-leaf displays as well as several variations including back-to-back stem-and-leaf displays and stretched stem-and-leaf displays. I also discuss the role of rounding and cutting data when dealing with large numbers or decimals, and … Read More

How to Interpret (and Create) Frequency Distribution Tables

In this video I explain frequency distribution tables and grouped frequency distribution tables for summarizing data. I explain each of the columns, including the variable measured, the frequency, the proportion and percentage of the data at each variable, and the cumulative frequency of scores. I also discuss choosing a class interval for a grouped frequency distribution and the difference between … Read More

n-1 Explained Conceptually

In this video I answer the common question of why we divide by n-1 when calculating variance from a sample, known as Bessel’s Correction. I focus on conceptual understanding of why this adjustment is needed and why n-1 is the appropriate adjustment on average, rather than making up a population and possible samples to illustrate this. I show why x-bar … Read More

Mean Absolute Deviation, Variance, & Standard Deviation

In this video I explain how to use mean deviation, mean absolute deviation, variance, and standard deviation to assess dispersion in interval or ratio level data. I discuss each concept and practice calculating with a sample set of data, and then consider why it’s so important to estimate the parameters of the population. Video Transcript Hi, I’m Michael Corayer and … Read More

How to Use Interquartile Range to Identify Outliers

In this video I explain how to use the interquartile range in order to identify possible outliers in a data set. This can be done using the interquartile range times 1.5 for moderate outliers, or times 3 for extreme outliers. This distance then creates a boundary below the 25th percentile and a boundary above the 75th percentile, creating a range … Read More

Inclusive vs. Exclusive Interquartile Range

In this video I explain the difference between calculating an exclusive interquartile range and an inclusive interquartile range. While we generally use an exclusive interquartile range with an even set of scores, when we have an odd-numbered set of scores we can choose which method we’d like to use. The inclusive method will tend to give us a slightly smaller … Read More

Dispersion: Range, Interquartile Range, & Semi-Interquartile Range

In this video I explain simple ways of describing the dispersion of data that can be ordered or ranked. These are the range, the interquartile range, and the semi-interquartile range. I also briefly describe a more precise method for calculating the range for a continuous variable, though this method is not common. I explain each of these measures of dispersion, … Read More