3 Properties of a Normal Distribution xThe mean, median, and mode are equalTell students that there are other bell shaped curves. The normal distributions are graphed with specific mathematical functions.Bell shaped and is symmetric about the meanThe total area that lies under the curve is one or 100%
4 Properties of a Normal Distribution Inflection pointInflection pointxAs the curve extends farther and farther away from themean, it gets closer and closer to the x-axis but never touches it.Tell students that there are other bell shaped curves. The normal distributions are graphed with specific mathematical functions. By identifying the points of inflection, students can roughly determine the standard deviation .The points at which the curvature changes are called inflection points. The graph curves downward between the inflection points and curves upward past the inflection points to the left and to the right.
5 Means and Standard Deviations Curves with different means, same standard deviation1011121314151617181920Curves with different means, different standard deviationsHave students find the means of 11, 15.5 and 21 for the top 3 curves. The standard deviation for each is one-half.For the lower 3 curves the means are 10, 15.5 and 21. The curve with the largest standard deviation is in the center. The one with the smallest is on the right.The middle curve on top has the same mean but different standard deviation from the middle curve on bottom.910111213141516171819202122
6 Empirical RuleAbout 68% of the area lies within 1 standard deviation of the mean68%About 95% of the area lies within 2 standard deviationsThis rule has been discussed earlier. Emphasize that there is still 0.3% of the distribution falling outside the 3 standard deviation limits.About 99.7% of the area lies within 3 standard deviations of the mean
7 Determining Intervals x188.8.131.52.184.108.40.206An instruction manual claims that the assembly time for a product is normally distributed with a mean of 4.2 hoursand standard deviation 0.3 hour. Determine theinterval in which 95% of the assembly times fall.A good chance to review probabilities. Find the probability an assembly time will be between 3.6 and Less than 4.5. Greater than 3.3 hours95% of the data will fall within 2 standard deviations of the mean.4.2 – 2 (0.3) = 3.6 and (0.3) = 4.8.95% of the assembly times will be between 3.6 and 4.8 hrs.
9 The Standard ScoreThe standard score, or z-score, represents the number of standard deviations a random variable x falls from the mean.The test scores for a civil service exam are normally distributed with a mean of 152 and a standard deviation of 7. Find the standard z-score for a person with a score of:(a) (b) (c) 152This concept was introduced in Chapter 2. The z-score is a measure of position.(a)(b)(c)
10 The Standard Normal Distribution The standard normal distribution has a mean of 0 and a standard deviation of 1.Using z-scores any normal distribution can be transformed into the standard normal distribution.When each value of a normal distribution is standardized, the standard normal distribution is produced. If students are using tables, they must standardize all values to find probabilities. If students are using a technology tool, this will not be necessary.z–4–3–2–11234
11 Cumulative AreasThetotalareaunderthe curveis one.z–3–2–1123As the value of z increases the cumulative area increases to one.The cumulative area is close to 0 for z-scores close to –3.49.The cumulative area for z = 0 isThe cumulative area is close to 1 for z-scores close to 3.49.
12 The probability that z is at most –1.25 is 0.1056. Cumulative AreasFind the cumulative area for a z-score of –220.127.116.116z–3–2–1123Read down the z column on the left to z = –1.25 and across to the column under .05. The value in the cell is , the cumulative area.Tell students it is a good idea to sketch the curve and indicate the area to be found.The probability that z is at most –1.25 is
13 Finding Probabilities To find the probability that z is less than a given value, read the cumulative area in the table corresponding to that z-score.Find P(z < –1.45).P (z < –1.45) =zThis is a “less than” example.–3–2–1123Read down the z-column to –1.4 and across to .05. The cumulative area is
14 Finding Probabilities To find the probability that z is greater than a given value, subtract the cumulative area in the table from 1.Find P(z > –1.24).0.10750.8925zThis is a “greater than” example. Students must compute the complementary area.–3–2–1123The cumulative area (area to the left) is So the area to the right is 1 – =P(z > –1.24) =
15 Finding Probabilities To find the probability z is between two given values, find the cumulative areas for each and subtract the smaller area from the larger.Find P(–1.25 < z < 1.17).This is a “between” example. Tell students to be sure to subtract the smaller area from the larger area since areas (and probabilities) cannot be negative.z–3–2–11231. P(z < 1.17) =2. P(z < –1.25) =3. P(–1.25 < z < 1.17) = – =
16 SummaryTo find the probability that z is lessthan a given value, read thecorresponding cumulative area.z-3-2-1123To find the probability is greater than a given value, subtract the cumulative area in the table from 1.Using the cumulative density function, the calculation of probabilities is greatly simplified to three possibilities. If you are using a 0-to z approach, skip these slides. With technologies use the CDF command to calculate cumulative densities.-3-2-1123zTo find the probability z is between two given values, find the cumulative areas for each and subtract the smaller area from the larger.z-3-2-1123
18 Probabilities and Normal Distributions If a random variable, x is normally distributed, the probability that x will fall within an interval is equal to the area under the curve in the interval.IQ scores are normally distributed with a mean of 100 and a standard deviation of 15. Find the probability that a person selected at random will have an IQ score less than 115.Recall that in a discrete probability distribution, we could use the area of the bar in the probability histogram to obtain the probability of the event. Here we can only find the probability that x will lie in a given interval.100115To find the area in this interval, first find the standard score equivalent to x = 115.
19 Probabilities and Normal Distributions Find P(x < 115).100115Standard Normal DistributionSAMESAMEThe area is the sameFind P(z < 1).1P(z < 1) = , so P(x <115) =
20 Application 0.8944 – 0.0475 = 0.8469 Normal Distribution Monthly utility bills in a certain city are normally distributed with a mean of $100 and a standard deviation of $12. A utility bill is randomly selected. Find the probability it is between $80 and $115.Normal DistributionP(80 < x < 115)P(–1.67 < z < 1.25)– =The probability a utility bill is between $80 and $115 is
22 From Areas to z-Scores z –1 1 2 3 4 0.9803 –4 –3 –2 Find the z-score corresponding to a cumulative area ofz = 2.06 correspondsroughly to the98th percentile.0.9803Be sure to emphasize that here, the area is given. Tell students to choose the z score closest to the given area. The only exception is if the area falls exactly at the midpoint between two z-scores, use the midpoint of the z=scores.–4–3–2–11234zLocate in the area portion of the table. Read the values at the beginning of the corresponding row and at the top of the column. The z-score is 2.06.
23 Finding z-Scores from Areas Find the z-score corresponding to the 90th percentile..90zThe closest table area is The row heading is 1.2 and column heading is .08. This corresponds to z = 1.28.A z-score of 1.28 corresponds to the 90th percentile.
24 Finding z-Scores from Areas Find the z-score with an area of .60 falling to its right..40.60zzWith .60 to the right, cumulative area is .40. The closest area is The row heading is 0.2 and column heading is .05. The z-score is 0.25.A z-score of 0.25 has an area of .60 to its right. It also corresponds to the 40th percentile
25 Finding z-Scores from Areas Find the z-score such that 45% of the area under the curve falls between –z and z..275.275.45–zzThe area remaining in the tails is .55. Half this area isin each tail, so since .55/2 = .275 is the cumulative area for the negative z value and = .725 is the cumulative area for the positive z. The closest table area is and the z-score is The positive z score is 0.60.Because the normal distribution is symmetric, the z scores will have the same absolute value. As a result, you can find one z-score and use its opposite for the other.
26 From z-Scores to Raw Scores To find the data value, x when given a standard score, z:The test scores for a civil service exam are normally distributed with a mean of 152 and a standard deviation of 7. Find the test score for a person with a standard score of:(a) (b) – (c) 0(a) x = (2.33)(7) =Show students that the formula given is equivalent to the z-score formula. Some students prefer to use only one formula and others like to use both. Have students work these through before displaying the answers. Emphasize the meaning of z-scores. A z-score of 2.33 is a 2.33 standard deviations above the mean.(b) x = (–1.75)(7) =(c) x = (0)(7) = 152
27 Finding Percentiles or Cut-off Values Monthly utility bills in a certain city are normally distributed with a mean of $100 and a standard deviation of $12. What is the smallest utility bill that can be in the top 10% of the bills?$ is the smallestvalue for the top 10%.90%10%zStudents find these “cut-off” problems easier if they think in terms of percentiles, which in turn are interpreted as cumulative areas.Find the cumulative area in the table that is closest to (the 90th percentile.) The area corresponds to a z-score of 1.28.To find the corresponding x-value, usex = (12) =
28 The Central Limit Theorem Section 5.5The Central Limit Theorem
29 Sampling Distributions A sampling distribution is the probability distribution of a sample statistic that is formed when samples of size n are repeatedly taken from a population. If the sample statistic is the sample mean, then the distribution is the sampling distribution of sample means.SampleSampleSampleSampleSampleSampleEach sample has the same n. Emphasize that sample means will vary from one sample to another but are not expected to be too far from the population mean. Other statistics such as the sample variance have their own sampling distributions that will be studied later.The sampling distribution consists of the values of the sample means,
30 The Central Limit Theorem If a sample n 30 is taken from a population withany type distribution that has a mean =and standard deviation =xthe sample means will have a normal distributionThis theorem is the foundation for inferential statistics. As long as the sample has at least 30 values, the sampling distribution of the mean will be norm.al. The center of the sampling distribution is the same as the center of the distribution of individual values. The variation is smaller. The larger the sample size, the smaller the variation will be.and standard deviation
31 The Central Limit Theorem If a sample of any size is taken from a population with a normal distribution with mean = and standard deviation =xthe distribution of means of sample size n, will be normalwith a meanstandard deviationWhen the original population is normally distributed, the sample can be any size for a normal sampling distribution.
32 Application 69.2 mean Standard deviation The mean height of American men (ages 20-29) isinches. Random samples of 60 such men are selected. Find the mean and standard deviation (standard error) of the sampling distribution.69.2meanDistribution of means of sample size 60,will be normal.Standard deviation
33 Interpreting the Central Limit Theorem The mean height of American men (ages 20-29) is = 69.2”. If a random sample of 60 men in this age group is selected, what is the probability the mean height for the sample is greater than 70”? Assume the standard deviation is 2.9”.Since n > 30 the sampling distribution of will be normalmeanstandard deviationFind the z-score for a sample mean of 70:
34 Interpreting the Central Limit Theorem Although the probability that one man might be more than 70 inches tall is P(z>0.28) = =.3897, the probability that the mean of a sample of 60 men will be greater than 70 isz2.14There is a probability that a sample of 60 men will have a mean height greater than 70”.
35 Application Central Limit Theorem During a certain week the mean price of gasoline in California was $1.164 per gallon. What is the probability that the mean price for the sample of 38 gas stations in California is between $1.169 and $1.179? Assume the standard deviation = $0.049.Since n > 30 the sampling distribution of will be normalmeanstandard deviationCalculate the standard z-score for sample values of $1.169 and $1.179.
36 Application Central Limit Theorem P( 0.63 < z < 1.90)= –=z.631.90The probability is that the mean for the sample is between $1.169 and $1.179.
37 Normal Approximation to the Binomial Section 5.6Normal Approximation to the Binomial
38 Binomial Distribution Characteristics • There are a fixed number of independent trials. (n)• Each trial has 2 outcomes, Success or Failure.• The probability of success on a single trial is p and the probability of failure is q p + q = 1• We can find the probability of exactly x successes out of n trials. Where x = 0 or 1 or 2 … n.• x is a discrete random variable representing a count of the number of successes in n trials.The table for calculating probabilities is limited to specific values of p and values of n that do not exceed 20. This application will show how to calculate binomial probabilities when the table cannot be used and the binomial probability formula becomes too tedious. Even technology tools such as Minitab have limitations in calculating these probabilities.
39 Application34% of Americans have type A+ blood. If 500 Americans are sampled at random, what is the probability at least 300 have type A+ blood?Using techniques of Chapter 4 you could calculate the probability that exactly 300, exactly 301… exactly 500 Americans have A+ blood type and add the probabilities.Or…you could use the normal curve probabilities to approximate the binomial probabilities.Review the formulas for calculating the mean and standard deviation of a binomial distribution. These must be found in order to specify the normal distribution.If np 5 and nq 5, the binomial random variable x is approximately normally distributed with mean
40 Why Do We Require np 5 and nq 5? p = 0.25, q = .75np = nq = 3.7512345n = 20p = 0.25np = 5 nq = 1512We have to ensure a large enough sample size. The minimum size depends on n and on p as well. When p is closer to .5, the curve is more symmetric and we require a smaller sample to approximate the normal distribution.345678911112131415161718192n = 50p = 0.25np = 12.5nq = 37.51020304050
41 Binomial Probabilities The binomial distribution is discrete with a probability histogram graph. The probability that a specific value of x will occur is equal to the area of the rectangle with midpoint at x.If n = 50 and p = 0.25 findAdd the areas of the rectangles with midpoints at x = 14, x = 15, x = 16.= 0.2650.1110.0890.065141516
42 Correction for Continuity Use the normal approximation to the binomial to find141516The continuous interval from 13.5 to 16.5 has approximately the same area as the rectangles whose centers are 14, 15 and 16.Values for the binomial random variable x are 14, 15 and 16.
43 Correction for Continuity Use the normal approximation to the binomial to find141516The continuous interval from 13.5 to 16.5 has approximately the same area as the rectangles whose centers are 14, 15 and 16.The interval of values under the normal curve isTo ensure the boundaries of each rectangle are included in the interval, subtract 0.5 from a left-hand boundary and add 0.5 to a right-hand boundary.
44 Normal Approximation to the Binomial Use the normal approximation to the binomial to find.Find the mean and standard deviation using binomial distribution formulas.Adjust the endpoints to correct for continuity PThe normal probability can be used to approximate the discrete binomial probability.Convert each endpoint to a standard score.
45 Application The binomial phrase of “fewer than 140” means A survey of Internet users found that 75% favored government regulations of “junk” . If 200 Internet users are randomly selected, find the probability that fewer than 140 are in favor of government regulation.Since np = 150 5 and nq = 50 5 use the normal approximation to the binomial.Students will agree that it is extremely impractical to use the binomial formulas for calculating the probability of exactly 0, exactly 1, exactly 2…exactly 139 successes. It is often helpful to have students list the possible values (for example 0, 1, 2…139). This helps determine the interval. the reason for not having to adjust the left hand limit is that the area is almost 0 at the extremes of the curve.Using the TI-83, binomcdf(200, .75, 139) the binomial probability is given asIf n = 2000 however, the TI-83 gives a domain error message. This means to calculate the probability students will need to use the normal approximation for the binomial.The binomial phrase of “fewer than 140” means0, 1, 2, 3…139.Use the correction for continuity to translate to the continuous variable in the interval Find P( x < 139.5).
46 ApplicationA survey of Internet users found that 75% favored government regulations of “junk” . If 200 Internet users are randomly selected, find the probability that fewer than 140 are in favor of government regulation.Use the correction for continuity P(x < 139.5).Students will agree that it is extremely impractical to use the binomial formulas for calculating the probability of exactly 0, exactly 1, exactly 2…exactly 139 successes. It is often helpful to have students list the possible values (for example 0, 1, 2…139). This helps determine the interval. the reason for not having to adjust the left hand limit is that the area is almost 0 at the extremes of the curve.Using the TI-83, binomcdf(200, .75, 139) the binomial probability is given asIf n = 2000 however, the TI-83 gives a domain error message. This means to calculate the probability students will need to use the normal approximation for the binomial.P( z < -1.71) =The probability that fewer than 140 are in favor of government regulation is approximately
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