Describing Distributions With Numbers Chapter 12

Transcription

1 Describing Distributions With Numbers Chapter 12 May 1, 2013 What Do We Usually Summarize? Measures of Center. Percentiles. Measures of Spread. A Summary.

2 1.0 What Do We Usually Summarize? source: Prof. Morita in STAT 221 For one Quantitative Variable (a) Center Value The center of the distribution. ``typical value in a certain sense'' (b) Spread of distribution How variable are the values from one another? Measure how many/what proportion of values are above / below a given value.

3 2.0 Measures of Center Here are current G.P.A.s of 15 students from one section

4 2.1 The Arithmetic Mean The mean x (x-bar) of a set of observations is their average. To find the mean of n observations, add the values and divide by n. x = sum of observations n For the G.P.A. data calculate the arithmetic mean of the G.P.A. scores x =, 15 = , = The mean has the same units as the data points.

5 2.2 Example Averages can summarize large quantities of data effectively.

6 2.3 Example The figure below shows age-specific average diastolic blood pressure for men age 20 and over in HANES ( ). True or false? As men age, their diastolic BP increases until age 45 or so and then decreases. If false, how do you explain the pattern?

7 2.4 The Median The median is the mid-point of the distribution, the number such that (at least) half of the observations are at the median or bigger and (at least) half are at the median or smaller. For the G.P.A. data calculate the median of the G.P.A. scores: 1. Order the observations from smallest to largest: Find the data point that has at least 7.5 observations above and below it. The median has the same units as the data points.

8 2.5 Means, Medians and Histograms List: 1, 2, 2, 3 50% median mean 0% % List: 1, 2, 2, 5 0% % List: 1, 2, 2, 7 0%

9 2.6 The Mode The mode is the most frequently occurring value in the data set. Here are G.P.A.s of 15 students from one section What is the mode? The mode has the same units as the data points.

10 3.0 Percentiles Definition The cth percentile of a distribution is defined so that (at least) c% of the observations are at or below it and (at least) (100-c)% of the observations are at or above it. Ex. SAT Score: If you scored in the 83rd percentile, this means? The median is the 50th percentile of a distribution. The 25th percentile of a distribution is called the lower quartile Q1. The 75th percentile of a distribution is called the upper quartile Q3.

11 3.1 Calculating Percentiles Back to the G.P.A Median = 3.3 (shown in box). Q1 =? Q3 =?

12 3.2 The Five Number Summary Definition The five number summary of a distribution consists of the smallest observation, the first quartile, the median, the third quartile, and the largest observation, written in order from smallest to largest. These five numbers offer a complete summary of a distribution. It is typically represented as a box-and-whisker plot.

13 3.3 The Box-and-Whisker Plot X A central box spans the quartiles. A line in the box marks the median. Sometimes the mean is marked by a cross. Lines extend from the box to the smallest and largest observation. Or they can extend to some other percentile (say 2.5th and 95th).

14 3.4 Learning from Box Plots source: W. Gray Number of Hurricanes in in wet and dry years in W. Africa 14 hurricanes x x Compare the medians. Assess skewness. Assess spread of middle 50% of data. 2 0 dry west.africa wet Does the data support the hypothesis that wet years tend to have more hurricanes?

15 4.0 Measures of Spread Range = Maximum - Minimum. Inter-quartile range (I.Q.R.) = Q3 - Q1. The I.Q.R. is the range of the middle 50% of a distribution. Some people call a data point an outlier if it is more than 1.5 times I.Q.R below Q1 or above Q I.Q.R. rule Standard Deviation (S.D.) Definition The standard deviation measures the average distance (or deviation) of the observations from their arithmetic mean.

16 4.1 Calculating Standard Deviations Find the S.D. for this list of numbers: 2, -6, 12, 4, 3. Step 1: Find the average for the list of numbers. The answer is 3. Step 2: Find the deviation of each value from this average: -1, -9, 9, 1, 0. Step 3: The S.D. tells the average size of a deviation. Step 3.1: Square each deviation: 1, 81, 81, 1, 0. square Step 3.2: Calculate the average of this list but dividing by (n 1) instead of n: The answer is 41. mean Step 3.3: Take the square-root of 41. The answer is 6.4. root The standard deviation is 6.4. has the same units as the list of numbers

17 4.2 Interpreting Standard Deviations The standard deviation (S.D.) says how far numbers on a list are from their average (or mean). A majority (about 50% or more) of entries will be somewhere around one S.D. from the average. Very few will be more than two or three S.D.s away. Majority of observations Ave- 1 S.D Ave Ave+ 1 S.D. Almost all the observations Ave-2 S.D.s Ave Ave+ 2 S.D.s

18 4.3 Guesstimating Standard Deviations Each of the following lists has an average of 50. For which one is the standard deviation the biggest? smallest? 1. 0, 20, 40, 50, 60, 80, , 48, 49, 50, 51, 52, , 1, 2, 50, 98, 99, 100.

19 4.4 Example Below are sketches of histograms for three lists of numbers. Match the sketch with the description that fits. (i) ave 3.5, S.D. 1 (ii) ave 3.5, S.D. 0.5 iii) ave 3.5, S.D. 2 (iv) ave 2.5, S.D. 1 (v) ave 2.5, S.D. 0.5 (vi) ave 4.5, S.D (a) (b) (c)

20 4.5 Example Household size in the U.S. has a mean of 2.5 people approximately. Which of these numbers would be a good guess for the standard deviation? 0.14, 1.4 or 2?

21 4.6 Example The Public Health Service found that for boys age 11 in HANES2, the average height was 146 cm and the SD was 8 cm. 1. One boy was 170cm tall. He was above average by SDs. 2. If a boy was within 2 SDs of average height, the shortest he could have been is cm and the tallest is cm. 3. Here are the heights of 3 boys: 150cm, 130 cm and 165cm. Match the heights with the descriptions. A description may be used twice. unusually short about average unusually tall

22 4.7 A Quick and Dirty Calculation Consider a list with only two different numbers, a big one and a small one. (Each number can be repeated many times). In this case, the S.D. can be estimated using: ( big number small number ) fraction with fraction with big number small number. Find the S.D. of the list of numbers: 1, -2, -2. Find the S.D. of the list of numbers: -1, -1, -1, 1. Can you use the short cut to calculate the standard deviation of the list: 1, 2, 3, 4?

23 4.8 A Summary To report the average and standard deviation, use the following language: In our data, the variable name here tends to be around average here, give or take standard deviation here. Advice Use means and standard deviations to summarize distributions that are roughly symmetric and with no outliers. Use the five number summary otherwise.