Key Terms

box plot
a graph that gives a quick picture of the middle 50 percent of the data
first quartile
the value that is the median of the lower half of the ordered data set
frequency
the number of times a value of the data occurs
frequency polygon
a data display that looks like a line graph but uses intervals to display ranges of large amounts of data
frequency table
a data representation in which grouped data are displayed along with the corresponding frequencies
histogram
a graphical representation in x-y form of the distribution of data in a data set; x represents the data and y represents the frequency, or relative frequency; the graph consists of contiguous rectangles
interquartile range
or IQR, is the range of the middle 50 percent of the data values; the IQR is found by subtracting the first quartile from the third quartile
interval
also called a class interval; an interval represents a range of data and is used when displaying large data sets
mean
a number that measures the central tendency of the data; a common name for mean is average.

 
The term mean is a shortened form of arithmetic mean. By definition, the mean for a sample (denoted by x¯x) is x¯ = Sum of all values in the sampleNumber of values in the samplex¯ = Sum of all values in the sampleNumber of values in the sample, and the mean for a population (denoted by μ) is μ=Sum of all values in the populationNumber of values in the populationμ=Sum of all values in the populationNumber of values in the population
median
a number that separates ordered data into halves; half the values are the same number or smaller than the median, and half the values are the same number or larger than the median

 
The median may or may not be part of the data.
midpoint
the mean of an interval in a frequency table
mode
the value that appears most frequently in a set of data
outlier
an observation that does not fit the rest of the data
paired data set
two data sets that have a one-to-one relationship so that
  • both data sets are the same size, and
  • each data point in one data set is matched with exactly one point from the other set
percentile
a number that divides ordered data into hundredths; percentiles may or may not be part of the data. The median of the data is the second quartile and the 50th percentile

 
The first and third quartiles are the 25th and the 75th percentiles, respectively.
quartiles
the numbers that separate the data into quarters; quartiles may or may not be part of the data; the second quartile is the median of the data
relative frequency
the ratio of the number of times a value of the data occurs in the set of all outcomes to the number of all outcomes
skewed
used to describe data that is not symmetrical; when the right side of a graph looks chopped off compared to the left side, we say it is skewed to the left

 
When the left side of the graph looks chopped off compared to the right side, we say the data are skewed to the right. Alternatively, when the lower values of the data are more spread out, we say the data are skewed to the left. When the greater values are more spread out, the data are skewed to the right.
standard deviation
a number that is equal to the square root of the variance and measures how far data values are from their mean; notation: s for sample standard deviation and σ for population standard deviation
variance
mean of the squared deviations from the mean, or the square of the standard deviation; for a set of data, a deviation can be represented as xx¯x¯ where x is a value of the data and x¯x¯ is the sample mean; the sample variance is equal to the sum of the squares of the deviations divided by the difference of the sample size and 1