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Central Tendency & Dispersion

Central Tendency & Dispersion. Types of Distributions: Normal, Skewed Central Tendency: Mean, Median, Mode Dispersion: Variance, Standard Deviation. DESCRIPTIVE STATISTICS are concerned with describing the characteristics of frequency distributions. Where is the center?

joanjeffery
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Central Tendency & Dispersion

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  1. Central Tendency & Dispersion • Types of Distributions: Normal, Skewed • Central Tendency: Mean, Median, Mode • Dispersion: Variance, Standard Deviation

  2. DESCRIPTIVE STATISTICSare concerned with describing the characteristics of frequency distributions • Where is the center? • What is the range? • What is the shape [of the distribution]?

  3. What is the range of test scores? Frequency TableTest Scores A: 30 (95 minus 65) When calculating mean, one must divide by what number? Observation Frequency (scores) (# occurrences) 65 1 70 2 75 3 80 4 85 3 90 2 95 1 A: 16 (total # occurrences)

  4. Frequency Distributions 4 3 2 1 Frequency (# occurrences) 65 70 75 80 85 90 95 Test Score

  5. NormallyDistributed Curve

  6. Voter Turnout in 50 States - 1980

  7. Skewed Distributions We say the distribution is skewed to the left  (when the “tail” is to the left) We say the distribution is skewed to the right  (when the “tail” is to the right)

  8. Voter Turnout in 50 States - 1940 Q: Is this distribution, positively or negatively skewed? A: Negatively Q: Would we say this distribution is skewed to the left or right? A: Left (skewed in direction of tail)

  9. Characteristics - Normal Distribution • It is symmetrical - half the values are to one side of the center (mean), and half the values are on the other side. • The distribution is single-peaked, not bimodal or multi-modal. • Most of the data values will be “bunched” near the center portion of the curve. As values become more extreme they become less frequent with the “outliers” being found at the “tails” of the distribution and are few in number. • The Mean, Median, and Mode are the same in a perfectly symmetrical normal distribution. • Percentage of values that occur in any range of the curve can be calculated using the Empirical Rule.

  10. EmpiricalRule

  11. Summarizing Distributions Two key characteristics of a frequency distribution are especially important when summarizing data or when making a prediction: • CENTRAL TENDENCY • What is in the “middle”? • What is most common? • What would we use to predict? • DISPERSION • How spread out is the distribution? • What shape is it?

  12. The MEASURES of Central Tendency • 3 measures of central tendencyare commonly used in statistical analysis - MEAN, MEDIAN, and MODE. • Each measure is designed to represent a “typical” value in the distribution. • The choice of which measure to use depends on the shape of the distribution (whether normal or skewed).

  13. Mean - Average • Most common measure of central tendency. • Is sensitive to the influence of a few extreme values (outliers), thus it is not always the most appropriate measure of central tendency. • Best used for making predictions when a distribution is more or less normal (or symmetrical). • Symbolized as: • x for the mean of a sample • μ for the mean of a population

  14. Median • Used to find middle value (center) of a distribution. • Used when one must determine whether the data values fall into either the upper 50% or lower 50% of a distribution. • Used when one needs to report the typical value of a data set, ignoring the outliers (few extreme values in a data set). • Example: median salary, median home prices in a market • Is a better indicator of central tendency than mean when one has a skewed distribution.

  15. Mode • Used when the most typical (common) value is desired. • Often used with categorical data. • The mode is not always unique. A distribution can have no mode, one mode, or more than one mode. When there are two modes, we say the distribution is bimodal. EXAMPLES: • {1,0,5,9,12,8} - No mode • {4,5,5,5,9,20,30} – mode = 5 • {2,2,5,9,9,15} - bimodal, mode 2 and 9

  16. Measures of Variability • Central Tendency doesn’t tell us everything Dispersion/Deviation/Spread tells us a lot about how the data values are distributed. • We are most interested in: • Standard Deviation (σ) and • Variance (σ2)

  17. Why can’t the mean tell us everything? • Mean describes the average outcome. • The question becomes how good a representation of the distribution is the mean? How good is the mean as a description of central tendency -- or how accurate is the mean as a predictor? • ANSWER -- it depends on the shape of the distribution. Is the distribution normal or skewed?

  18. Dispersion • Once you determine that the data of interest is normally distributed, ideally by producing a histogram of the values, the next question to ask is: How spread out are thevalues about the mean? • Dispersion is a key concept in statistical thinking. • The basic question being asked is how much do the values deviate from the Mean? The more “bunched up” around the mean the better your ability to make accurate predictions.

  19. Means • Consider these means for hours worked day each day: X = {7, 8, 6, 7, 7, 6, 8, 7} X = (7+8+6+7+7+6+8+7)/8 X = 7 Notice that all the data values are bunched near the mean. Thus, 7 would be a pretty good prediction of the average hrs. worked each day. X = {12, 2, 0, 14, 10, 9, 5, 4} X = (12+2+0+14+10+9+5+4)/8 X = 7 The mean is the same for this data set, but the data values are more spread out. So, 7 is not a good prediction of hrs. worked on average each day.

  20. Data is more spread out, meaning it has greater variability. Below, the data is grouped closer to the center, less spread out, or smaller variability.

  21. How well does the mean represent the values in a distribution? • The logic here is to determine how much spread is in the values. How much do the values "deviate" from the mean? Think of the mean as the true value, or as your best guess. If every X were very close to the Mean, the Mean would be a very good predictor. • If the distribution is very sharply peaked then the mean is a good measure of central tendency and if you were to use the Mean to make predictions you would be correct or very close much of the time.

  22. What Does it Mean? • On Average, each value is two units away from the mean. Is it Really that Easy? • No! • Absolute values are difficult to manipulate algebraically • Absolute values cause enormous problems for calculus (Discontinuity) • We need something else…

  23. Variance and Standard Deviation • Instead of taking the absolute value, we square the deviations from the mean. This yields a positive value. • This will result in measures we call the Variance and the Standard Deviation Sample - Population - s Standard Deviation σ Standard Deviation s2 Variance σ2 Variance

  24. Skewed distributions https://www.youtube.com/watch?v=diYKIsXPAb8&nohtml5=False First https://learnzillion.com/lesson_plans/4866-find-population-percentages-using-z-scores-and-tables/lesson Second https://learnzillion.com/lesson_plans/5361-find-population-percentages-using-technology/ Third https://learnzillion.com/lesson_plans/5638-solve-problems-around-normal-distributions-using-a-graphing-calculator/

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