lecture 25 - data-8.github.iodata8.org/materials-sp18/lec/lec25pdf.pdf · lecture 25 center and...
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DATA 8Spring 2018
Slides created by John DeNero ([email protected]) and Ani Adhikari ([email protected])
Lecture 25Center and Spread
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Announcements
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Chance versus Confidence
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Is This What a CI Means?An approximate 95% confidence interval for the average age of the mothers in the population is (26.9, 27.6) years.
True or False:● There is a 0.95 probability that the average age of
mothers in the population is in the range 26.9 to 27.6 years.
Answer: False. The average age of the mothers in the population is unknown but it’s a constant. It’s not random. No chances involved.
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Confidence Intervals For Testing
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Using a CI for Testing● Null hypothesis: Population average = x● Alternative hypothesis: Population average ≠ x● Cutoff for P-value: p%● Method:
○ Construct a (100-p)% confidence interval for the population average
○ If x is not in the interval, reject the null○ If x is in the interval, can’t reject the null
(Demo)
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Center and Spread
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Questions ● How can we quantify natural concepts like “center” and
“variability”?
● Why do many of the empirical distributions that we generate come out bell shaped?
● How is sample size related to the accuracy of an estimate?
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Average
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The Average (or Mean)Data: 2, 3, 3, 9 Average = (2+3+3+9)/4 = 4.25● Need not be a value in the collection● Need not be an integer even if the data are integers● Somewhere between min and max, but not necessarily
halfway in between● Same units as the data● Smoothing operator: collect all the contributions in one
big pot, then split evenly
(Demo)
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Discussion QuestionCreate a data set that has this histogram. (You can do it with a short list of whole numbers.)
What are its median and mean?
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Discussion QuestionAre the medians of these two distributions the same or different? Are the means the same or different? If you say “different,” then say which one is bigger.
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● Mean: Balance point of the histogram
● Median: Half-way point of data; half the area of histogram is on either side of median
● If the distribution is symmetric about a value, then that value is both the average and the median.
● If the histogram is skewed, then the mean is pulled away from the median in the direction of the tail.
Comparing Mean and Median
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Discussion QuestionWhich is bigger?
(a) mean
(b) median
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Standard Deviation
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Defining VariabilityPlan A: “biggest value - smallest value”● Doesn’t tell us much about the shape of the distribution
Plan B:● Measure variability around the mean● Need to figure out a way to quantify this
(Demo)
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How Far from the Average?● Standard deviation (SD) measures roughly how far the
data are from their average
● SD = root mean square of deviations from average 5 4 3 2 1
● SD has the same units as the data
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Why Use the SD?
● The first reason:No matter what the shape of the distribution,the bulk of the data are in the range “average ± a few SDs”
There are two main reasons.
● The second reason:Coming up in the next lecture.
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Chebyshev's Inequality
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The Mathematician’s Name● Chebyshev● Chebychev● Chebishov● Čebyšev● Tchebichev● Tchebicheff● Tschebyscheff● Tschebyschew● Чебышёв
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How Big are Most of the Values?No matter what the shape of the distribution,the bulk of the data are in the range “average ± a few SDs”
Chebyshev’s InequalityNo matter what the shape of the distribution,the proportion of values in the range “average ± z SDs” is
at least 1 - 1/z²
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Chebyshev’s BoundsRange Proportion
average ± 2 SDs at least 1 - 1/4 (75%)
average ± 3 SDs at least 1 - 1/9 (88.888…%)
average ± 4 SDs at least 1 - 1/16 (93.75%)
average ± 5 SDs at least 1 - 1/25 (96%)
No matter what the distribution looks like