The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL STOR 155 Introductory Statistics Lecture 5 Density Curves and Normal Distributions I 5 17 10 Lecture 5 1 A problem about Standard Deviation A variable has 5 possible values The mean is 125 and the standard deviation is 25 If a sixth number of 125 is added in what is the new standard deviation 5 17 10 Lecture 5 2 To describe distribution of variable X Plot your data Stemplot histogram boxplot Look for overall pattern shapes striking deviations outliers Calculate appropriate numerical summaries about center and spread Mean median mode Range percentiles quartiles standard deviation Five number summary boxplot IQR and outliers Make things easier with a smooth curve 5 17 10 Lecture 5 3 5 17 10 Lecture 5 4 Density Curve A Math Approximation To calculate probabilities we define a probability density function f x The curve that plots f x is called the corresponding density curve f x satisfies f x 0 The total area under the curve representing f x equals 1 5 17 10 Lecture 5 5 Density Curves Describe the overall shape of distributions Idealized mathematical models for distributions Show patterns that are accurate enough for practical purposes Always on or above the horizontal axis The total area under the curve is exactly 1 Areas under the curve represent relative frequencies of observations 5 17 10 Lecture 5 6 5 17 10 Lecture 5 7 Histograms vs Density Curves Histograms show either frequencies counts or relative frequencies proportions in each class interval Density curves show the proportion of observations in any region by areas under the curve You can think of density curve as an approximation to refined histograms when there are a lot of observations 5 17 10 Lecture 5 8 Histogram vs Density Curve 5 17 10 Lecture 5 9 Center of a Density Curve The mode of a distribution is the point where the curve is the highest Highest Point The median is the point where half of the area under the curve lies on the left and the other half on the right Equal Areas Cut off Point The mean is the point at which the curve would balance if made out of solid material Balance Point 5 17 10 Lecture 5 10 Mean of a Density Curve 5 17 10 Lecture 5 11 Spread of a Density Curve Quartiles can be found by dividing the area under the curve into four equal parts of the area is to the left of the 1st quartile Q1 of the area is to the left of the 3rd quartile Q3 The standard deviation of a density curve is denoted by Not easy to calculate integration in calculus needed 5 17 10 Lecture 5 12 5 17 10 Lecture 5 13 Normal Distribution Graphically speaking a Normal Distribution is a distribution that has a symmetric unimodal and bell shaped density curve The mean and standard deviation completely specify the curve The mean median and mode are the same 5 17 10 Lecture 5 14 The height of a normal density curve at any point x is given by 2 1 12 x f x e 2 is the mean is the standard deviation 5 17 10 Lecture 5 15 Change of Curvature 5 17 10 Lecture 5 16 Example The normal distribution is the most important distribution in Statistics Typical normal curves with different sigma standard deviation values are shown below 5 17 10 Lecture 5 17 Examples with approximate Normal distributions Height Weight IQ scores Standardized test scores Body temperature Repeated measurement of same quantity 5 17 10 Lecture 5 18 The 68 95 99 7 Rule 5 17 10 Lecture 5 2 3 19 The 68 95 99 7 Rule 5 17 10 Lecture 5 20 Example Young Women s Height The heights of young women are approximately normal with mean 64 5 inches and std dev 2 5 inches 5 17 10 Lecture 5 21 Example Young Women s Height of young women between 62 and 67 of young women lower than 62 or taller than 67 between 59 5 and 62 taller than 68 25 5 17 10 Lecture 5 22 Standardizing and z Scores an observation x comes from a distribution with mean and standard deviation The standardized value of x is defined as z x which is also called a z score A z score indicates how many standard deviations the original observation is away from the mean and in which direction Mean and S D of the distribution of z 5 17 10 Lecture 5 23 Example Young Women s Height The heights of young women are approximately normal with mean 64 5 inches and std dev 2 5 inches In our class there is a female student who is 68 25 inches tall what is her z score 5 17 10 Lecture 5 24 Effects of Standardizing Standardizing is a linear transformation What are a and b Effects on shape center and spread The standardized values for any distribution always have mean 0 and standard deviation 1 Linear transformation normal into normal 5 17 10 Lecture 5 25 The standard normal distribution The standard normal distribution is the normal dist with mean 0 and standard deviation 1 denoted as N 0 1 N 0 1 can be treated as a benchmark Any normal distribution can be related to N 0 1 by a linear transformation Z N 0 1 What is the distribution for X a bZ 5 17 10 Lecture 5 26 Take Home Message Density curve Center spread Normal distributions and normal curves The 68 95 99 7 rule for normal distributions Standardizing observations The standard normal distribution 5 17 10 Lecture 5 27
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