# Lesson 20. Probability and Cumulative Distribution Functions

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1 Lesson 20 Probability and Cumulative Distribution Functions

2 Recall If p(x) is a density function for some characteristic of a population, then

3 Recall If p(x) is a density function for some characteristic of a population, then We also know that for any density function,

4 Recall We also interpret density functions as probabilities: If p(x) is a probability density function (pdf), then

5 Cumulative Distribution Function Suppose p(x) is a density function for a quantity. The cumulative distribution function (cdf) for the quantity is defined as Gives: The proportion of population with value less than x The probability of having a value less than x.

6 Example: A Spinner Last class: A spinner that could take on any value 0º x 360º. Density function: p(x) = 1/360 if 0 x 360, and 0 everywhere else.

7 Example: A Spinner Last class: A spinner that could take on any value 0º x 360º. Density function: p(x) = 1/360 if 0 x 360, and 0 everywhere else. CDF:

8 Example: A Spinner Density Function: Cumulative Distribution Function:

9 Properties of CDFs P(x) is the probability of values less than x If P(x) is the cdf for the age in months of fish in a lake, then P(10) is the probability a random fish is 10 months or younger. P(x) goes to 0 as x gets smaller: (In many cases, we may reach 0.)

10 Properties of CDFs Conversely, P(x) is non-decreasing. The derivative is a density function, which cannot be negative. Also, P(4) can t be less than P(3), for example.

11 Practice Life expectancy (in days) of electronic component has density function, for x 1, and p(x) = 0 for x < 1. a) Probability component lasts between 0 and 1 day? b) Probability it lasts between 0 and 10 days? c) More than 10 days? d) Find the CDF for the life expectancy.

12 Practice Life expectancy (in days) of electronic component has density function, for x 1, and p(x) = 0 for x < 1. a) Probability component lasts between 0 and 1 day? Zero!

13 Practice Life expectancy (in days) of electronic component has density function, for x 1, and p(x) = 0 for x < 1. b) Probability it lasts between 0 and 10 days?

14 Practice Life expectancy (in days) of electronic component has density function, for x 1, and p(x) = 0 for x < 1. c) More than 10 days?

15 Practice Life expectancy (in days) of electronic component has density function, for x 1, and p(x) = 0 for x < 1. c) More than 10 days?

16 Practice Life expectancy (in days) of electronic component has density function, for x 1, and p(x) = 0 for x < 1. d) Find the CDF for life expectancy. If x < 1, then If x 1, then

17 Practice Life expectancy (in days) of electronic component has density function, for x 1, and p(x) = 0 for x < 1. d) Find the CDF for life expectancy.

18 Example Someone claims this is the CDF for grades on the 2015 final exam. Probability a random student scored. 25 or lower? 1, or 100%. 50 or lower? 0.5, or 50%. Conclusion? They re lying! This cannot be a cumulative distribution function! (It decreases.)

19 Relating the CDF and DF According the FTC version 2, since then P'(x) = p(x).,

20 Relating the CDF and DF According the FTC version 2, since then P'(x) = p(x). So the density function is the derivative, or rate of change, of the cumulative distribution function.,

21 Example Sketch the density function for the cdf shown.

22 Example Sketch the density function for the cdf shown. Slope?

23 Example Sketch the density function for the cdf shown. Slope? 0

24 Example Sketch the density function for the cdf shown. Slope? 0 Slope 1/2

25 Example Sketch the density function for the cdf shown. Slope 0 Slope? 0 Slope 1/2

26 Example Sketch the density function for the cdf shown. Slope 0 Slope? 0 Slope 1/2 Slope 1/2

27 Example Sketch the density function for the cdf shown. Slope 0 Slope 0 Slope? 0 Slope 1/2 Slope 1/2

28 Example Sketch the density function for the cdf shown. Slope 0 Slope 0 Slope? 0 Slope 1/2 Slope 1/2 Density Function

29 DF vs. CDF You must know which is which. We work differently with density functions than with cumulative distribution functions.

30 Another Example Suppose the cumulative distribution function for the height of trees in a forest (in feet) is given by Find the height, x for which exactly half the trees are taller than x feet, and half the trees are shorter than x. In case - Quadratic Formula: x =!b ± b 2! 4ac 2a

31 Another Example Find the height, x for which exactly half the trees are taller than x feet, and half the trees are shorter than x. 0.1x! x 2 = 0.5!0.0025x x! 0.5 = 0!0.0025(x 2! 40x + 200) = 0 using quadratic : x = 20 ±10 2 = 20!10 2 " 5.86 ft

32

33

34 Assume p(x) is the density function " 1 \$,!!!if!1! x! 3 p(x) = # 2 % \$ 0,!!!if!x < 1!!or!!x > 3 Let P(x) represent the corresponding Cumulative Distribution function. Then (a) For the density function given above, what is the cumulative distribution function P(x) for values of x between 1 and 3? Example (1) (2) (b) P(5) =? (c) Choose the correct pair of graphs for p(x)!and!p(x). (3) (4)

35 In-class Assignment The time to conduct a routine maintenance check on a machine has a cumulative distribution function P(t), which gives the fraction of maintenance checks completed in time less than or equal to t minutes. Values of P(t) are given in the table. t, minutes P(t), fraction completed a. What fraction of maintenance checks are completed in 15 minutes or less? b. What fraction of maintenance checks take longer than 30 minutes? c. What fraction take between 10 and 15 minutes?

36 In-class Assignment The time to conduct a routine maintenance check on a machine has a cumulative distribution function P(t), which gives the fraction of maintenance checks completed in time less than or equal to t minutes. Values of P(t) are given in the table. t, minutes P(t), fraction completed a. What fraction of maintenance checks are completed in 15 minutes or less? 21% b. What fraction of maintenance checks take longer than 30 minutes? 2% c. What fraction take between 10 and 15 minutes? 13%

37 Lesson 21 Mean and Median

38 The Median The median is the halfway point: Half the population has a lower value, and half a higher value. If p(x) is a density function, then the median of the distribution is the point M such that M # p(x)dx = 0.5!"

39 Example Suppose an insect life span of months, and 0 elsewhere. What s the median life span? We need Then solve for M: months

40 Median From CDF If we have a CDF P(x), the median M is the point where P(M) = 0.5 (This of course says 1/2 of the population has a value less than M!)

41 Example CDF: P(t) = t 2, 0 t 1. What s the median of this distribution?

42 Example CDF: P(t) = t 2, 0 t 1. What s the median of this distribution?

43 The Mean What most people think of as the average: Add up n values and divide by n. How should we represent this for something represented by a density function p(x)?

44 The Mean We define the mean of a distribution given by density function p(x) to be

45 Example Previous insect population example: and p(x) = 0 elsewhere. What s the mean lifespan?

46 Example Previous insect population example: and p(x) = 0 elsewhere. What s the mean lifespan? 8 months. The mean is slightly below the median. This means

47 Previous insect population example: What s the mean lifespan? Example and p(x) = 0 elsewhere. 8 months. The mean is slightly below the median. This means more than half the insects live longer than the mean lifespan.

48 From Another Example, p(x) = 0.1! 0.005x mean = " Example 2 # x p(x)dx = # x( 0.1! 0.005x)dx =!" # 0.1x! 0.005x 2 dx = 0.05x 2! x = 6.64 ft 20

49 Geometry Turns out: the mean is the point where the graph of the distribution would balance if we cut it out.

50 Example Find the mean and median for and zero elsewhere. Median: Solve for M

51 Example Find the mean and median for and zero elsewhere. Mean: Evaluate

52 Example Find the mean and median for and zero elsewhere. mean median

53 The Normal Distribution A special distribution:

54 The Normal Distribution A special distribution:

55 The Normal Distribution Here s an example with µ = 0 and σ = 1: The distribution is symmetric about x = µ. So the mean is µ, and so is the median. σ is called the standard deviation, and describes how spread out the curve is.

56 The Normal Distribution Unfortunately, has no elementary antiderivative. So we must use numerical means to evaluate integrals involving the normal distribution.

57 Example Suppose a density function for number of people (in millions) on the internet at one time were 2 What are the mean and median?

58 Example Suppose a density function for number of people (in millions) on the internet at one time were 2 What are the mean and median? Here, µ = 10, so both the mean and median are 10 million people.

59 Example Suppose a density function for number of people (in millions) on the internet at one time were 2 What is the standard deviation?

60 Example Suppose a density function for number of people (in millions) on the internet at one time were 2 What is the standard deviation? Here, we see that σ = 2, so 2 million people.

61 Example Suppose a density function for number of people (in millions) on the internet at one time were 2 What is the standard deviation? Here, we see that σ = 2, so 2 million people.

62 Example Suppose a density function for number of people (in millions) on the internet at one time were 2 Write an integral for the fraction of time between 8 and 12 million people are on the internet.

63 Example Suppose a density function for number of people (in millions) on the internet at one time were 2 Write an integral for the fraction of time between 8 and 12 million people are on the internet. 2

64 Evaluating the Integral How to calculate the required integral? 2

65 Evaluating the Integral How to calculate the required integral? 2 We could use our Simpson s rule calculator:

66 Evaluating the Integral How to calculate the required integral? 2 We could use our Simpson s rule calculator: n!x Result

67 Evaluating the Integral How to calculate the required integral? 2 We could use our Simpson s rule calculator: n!x Result Seems to converge to about 0.68.

68 In Class Assignment Find the mean and median of the distribution with density function: (calculations should be to 2 decimal places) p(x) = 1 4 x3 for 0 < x < 2!and!0!elsewhere

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