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Published bySabrina Hoover Modified over 9 years ago
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Measures of Central Tendency 3.1
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● Analyzing populations versus analyzing samples ● For populations We know all of the data Descriptive measures of populations are called parameters Parameters are often written using Greek letters ( μ ) ● For samples We know only part of the entire data Descriptive measures of samples are called statistics Statistics are often written using Roman letters ( ) PARAMETER VS. STATISTIC
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● The arithmetic mean of a variable is often what people mean by the “average” … add up all the values and divide by how many there are ● Compute the arithmetic mean of 6, 1, 5 ● Add up the three numbers and divide by 3 (6 + 1 + 5) / 3 = 4.0 ● The arithmetic mean is 4.0 ARITHMETIC MEAN
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● The arithmetic mean is usually called the mean ● For a population … the population mean Is computed using all the observations in a population Is denoted μ Is a parameter ● For a sample … the sample mean Is computed using only the observations in a sample Is denoted Is a statistic MEAN
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Whole Class (Population) Height in Inches Find the arithmetic mean 5 Students (Sample…statistic) Height in Inches Find the arithmetic mean LET’S TRY IT
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● The median of a variable is the “center” ● When the data is sorted in order, the median is the middle value ● The calculation of the median of a variable is slightly different depending on If there are an odd number of points, or The middle number If there are an even number of points Take the 2 middle numbers and find the mean MEDIAN
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● An example with an odd number of observations (5 observations) ● Compute the median of 6, 1, 11, 2, 11 ● Sort them in order 1, 2, 6, 11, 11 ● The middle number is 6, so the median is 6 MEDIAN
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● An example with an even number of observations (4 observations) ● Compute the median of 6, 1, 11, 2 ● Sort them in order 1, 2, 6, 11 ● Take the mean of the two middle values (2 + 6) / 2 = 4 ● The median is 4 MEDIAN
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● The mode of a variable is the most frequently occurring value ● Find the mode of 6, 1, 2, 6, 11, 7, 3 ● The values are 1, 2, 3, 6, 6, 7, 11 ● The value 6 occurs twice, all the other values occur only once ● The mode is 6 MODE
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● Qualitative data Values are one of a set of categories Cannot add or order them … the mean and median do not exist The mode is the only one of these three measurements that exists ● Find the mode of blue, blue, blue, red, green ● The mode is “blue” because it is the value that occurs the most often WEIRD MODE
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● Quantitative data The mode can be computed but sometimes it is not meaningful Sometimes each value will only occur once (which can often happen with precise measurements) ● Find the mode of 5.1, 6.6, 6.8, 9.3, 1.9 ● Each value occurs only once ● The mode is not a meaningful measurement ● Mode is what is used in elections! MODE (NO REPEATS)
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The mean and the median are often different This difference gives us clues about the shape of the distribution Is it symmetric? Is it skewed left? Is it skewed right? Are there any extreme values? SHAPE
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Symmetric – the mean will usually be close to the median Skewed left – the mean will usually be smaller than the median Skewed right – the mean will usually be larger than the median SHAPE
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● If a distribution is symmetric, the data values above and below the mean will balance The mean will be in the “middle” The median will be in the “middle” ● Thus the mean will be close to the median, in general, for a distribution that is symmetric SYMMETRIC
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● If a distribution is skewed left, there will be some data values that are larger than the others The mean will decrease The median will not decrease as much ● Thus the mean will be smaller than the median, in general, for a distribution that is skewed left SKEWED LEFT
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● If a distribution is skewed right, there will be some data values that are larger than the others The mean will increase The median will not increase as much ● Thus the mean will be larger than the median, in general, for a distribution that is skewed right SKEWED RIGHT
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Birth Weights 5.87.49.27.08.57.6 7.97.87.97.79.07.1 8.77.26.17.27.17.2 7.95.97.07.87.27.5 7.36.47.48.29.17.3 FINDING MEAN AND MEDIAN ON CALCULATOR Find the mean and median Make a Histogram to discuss the shape of the data How to sort data in lists
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Mean The center of gravity Useful for roughly symmetric quantitative data Median Splits the data into halves Useful for highly skewed quantitative data Mode The most frequent value Useful for qualitative data SUMMARY (IMPORTANT STUFF!)
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