Chapter 9


Bimodal distribution A polygon with two peaks. Box and whisker (plot)


A means of representing the distribution of EAP test scores.


Descriptive statistics


Statistical procedures which are useful for identifying groupings or distributions within a set of data.


Interquartile range


Interquartile range is the range of the central half of a set of data. When compared to the range, the interquartile range is a more effective measure of central tendency, as it focuses on the spread of the middle half of the values.


Interval scale


Provides categories and ordering, and shows distance between points in that ordering. Notably, intervals have the same interpretation throughout (no absolute zero).


Kurtosis


The level of peakedness in symmetrical distributions in a frequency polygon.


Leptokurtic


A highly peaked polygon in a frequency polygon.


Mean


The average of a test score, often interpreted as an indicator of test difficulty or how a given group of students has performed.


Median


The midpoint value in a series of data and splits the data into two equal sets.


Mesokurtic


A middle peaked polygon (typical of normal distribution).


Mode


The mode is the most frequently occurring score.


Negatively skewed


A longer tail at the lower end and a peak at the upper end of the polygon.


Nominal scale


Provides names and categories only (no order implied).


Normal distribution


Used to describe a frequency polygon, takes the form of a bell curve. Normal distributions, which can vary in shape, appear in many naturally occurring situations, including in educational variables relevant to EAP, such as ability in different language skills.


Ordinal scale


Provides categories, but also provides the ordering or ranking of those categories (no information about distance between categories).


Platykurtic


A relatively flat polygon. Positively skewed


A longer tail at the upper end and a peak at the lower end of the polygon.


Range


The range shows the difference between the highest and lowest scores. It can be calculated by subtracting the lowest score from the highest score.


Ratio scale


Provides the intervals between points in the ordering of certain categories, but with more information because there is an interpretable zero and multiples of points along the scale make sense.


Semi-interquartile range The semi-interquartile range is the interquartile range divided by two (Q3 - Q1) ÷ 2. It is a measure of variability which is frequently used for data sets which are not normally distributed.


Skewness


A term used to refer to distributions which show a form of asymmetry.


Standard deviation


The standard deviation shows how much, on average, test scores vary or deviate from the mean.


Glossary


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