# Histograms and density curves

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1 Histograms and density curves What s in our toolkit so far? Plot the data: histogram (or stemplot) Look for the overall pattern and identify deviations and outliers Numerical summary to briefly describe center and spread A new idea: If the pattern is sufficiently regular, approximate it with a smooth curve. Any curve that is always on or above the horizontal axis and has total are underneath equal to one is a density curve. Area under the curve in a range of values indicates the proportion of values in that range. Come in a variety of shapes, but the normal family of familiar bell-shaped densities is commonly used. Remember the density is only an approximation, but it simplifies analysis and is generally accurate enough for practical use. The Normal Distrbution, Jan 9,

2 Examples Sulfur oxide (in tons) Shaded area of histogram: Sulfur oxide (in tons) Shaded area under the curve: Sulfur oxide (in tons) Waiting time between eruptions (min) The Normal Distrbution, Jan 9,

3 Median and mean of a density curve Median: The equal-areas point with 50% of the mass on either side. Mean: The balancing point of the curve, if it were a solid mass. Note: The mean and median of a symmetric density curve are equal. The mean of a skewed curve is pulled away from the median in the direction of the long tail. The mean and standard deviation of a density are denoted µ and σ, rather than x and s, to indicate that they refer to an idealized model, and not actual data. The Normal Distrbution, Jan 9,

4 Normal distributions: N (µ, σ) The normal distribution is symmetric, single-peaked, bell-shaped. The density curve is given by ( ) f(x) = 1 exp 1 2πσ 2 2σ 2(X µ)2. It is determined by two parameters µ and σ: µ is the mean (also the median) σ is the standard deviation Note: The point where the curve changes from concave to convex is σ units from µ in either direction. The Normal Distrbution, Jan 9,

5 The rule About 68% of the data fall inside (µ σ, µ + σ). About 95% of the data fall inside (µ 2σ, µ + 2σ). About 99.7% of the data fall inside (µ 3σ, µ + 3σ). The Normal Distrbution, Jan 9,

6 Example Scores on the Wechsler Adult Intelligence Scale (WAIS) for the 20 to 34 age group are approximately N(110, 25). About what percent of people in this age group have scores above 110? About what percent have scores above 160? In what range do the middle 95% of all scores lie? The Normal Distrbution, Jan 9,

7 Standardization and z-scores Linear transformation of normal distributions: X N (µ, σ) a X + b N (a µ + b, a σ) In particular it follows that X µ σ N (0, 1). N (0, 1) is called standard normal distribution. For a real number x the standardized value or z-score z = x µ σ tells how many standard deviations x is from µ, and in what direction. Standardization enables us to use a standard normal table to find probabilities for any normal variable. For example: What is the proportion of N(0, 1) observations less than 1.2? What is the proportion of N(3, 1.5) observations greater than 5? What is the proportion of N(10, 5) observations between 3 and 9? The Normal Distrbution, Jan 9,

8 Normal calculations Standard normal calculations 1. State the problem in terms of x. 2. Standardize: z = x µ σ. 3. Look up the required value(s) on the standard normal table. 4. Reality check: Does the answer make sense? Backward normal calculations We can also calculate the values, given the probabilities: If MPG N (25.7, 5.88), what is the minimum MPG required to be in the top 10%? Backward normal calculations 1. State the problem in terms of the probability of being less than some number. 2. Look up the required value(s) on the standard normal table. 3. Unstandardize, i.e. solve z = x µ σ for x. The Normal Distrbution, Jan 9,

9 Example Suppose X N (0, 1). P(X 2) =? P(X > 2) =? P( 1 X 2) =? Find the value z such that P(X z) = 0.95 P(X > z) = 0.99 P( z X < z) = 0.68 P( z X < z) = 0.95 P( z X < z) = Suppose X N (10, 5). P(X < 5) =? P( 3 < X < 5) =? P( x < X < x) = 0.95 The Normal Distrbution, Jan 9,

10 Assessing Normality How to make a normal quantile plot 1. Arrange the data in increasing order. 2. Record the percentiles ( 1 n, 2 n,..., n n ). 3. Find the z-scores for these percentiles. 4. Plot x on the vertical axis against z on the horizontal axis. Use of normal quantile plots If the data are (approximately) normal, the plot will be close to a straight line. Systematic deviations from a straight line indicate a nonnormal distribution. Outliers appear as points that are far away from the overall patter of the plot. Sample Quantiles N(0, 1) Sample Quantiles Exp(1) Sample Quantiles U(0, 1) Theoretical Quantiles Theoretical Quantiles Theoretical Quantiles The Normal Distrbution, Jan 9,

11 Estimation The normal density is just one possible density curve. There are many others, some with compact mathematical formulas and many without. estimation software fits an arbitrary density to data to give a smooth summary of the overall pattern Velocity of galaxy (1000km/s) The Normal Distrbution, Jan 9,

12 Histogram How to scale a histogram? Easiest way to draw a histogram: qqually spaced bins counts on the vertical axis Frequency Sosa home runs Disadvantage: Scaling depends on number of observations and bin width. Scale histogram such that area of each bar corresponds to proportion of data: height = counts width total number Sosa home runs Proportion of data in interval (0, 10]: height width = = 0.2 = 20% Since n = 15 this corresponds to 3 observations. The Normal Distrbution, Jan 9,

13 curves n= n=2500 n= x x Proportion of data in (1,2]: #{x i : 1 < x i 2} n n x x 2 1 n f(x) dx Probability that a new observation X fall into [a, b] P(a X b) = b a f(x) dx = lim n #{x i : 1 < x i 2} n The Normal Distrbution, Jan 9,

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