# AP Statistics Curriculum 2007 Normal Prob

(Difference between revisions)
 Revision as of 23:49, 17 September 2008 (view source)IvoDinov (Talk | contribs) (→General Normal Distribution)← Older edit Revision as of 20:03, 5 November 2008 (view source)IvoDinov (Talk | contribs) (→Assessing Normality)Newer edit → Line 42: Line 42: ===Assessing Normality=== ===Assessing Normality=== How can we tell if data collected from a process or experiment we observe is normally distributed? There are several methods for ''checking normality'': How can we tell if data collected from a process or experiment we observe is normally distributed? There are several methods for ''checking normality'': - * Symmetry: Are the [[AP_Statistics_Curriculum_2007_EDA_Center |mean and median]] of the dataset equal (Mean = Median)? Use the [[AP_Statistics_Curriculum_2007_Distrib_MeanVar | skewness measure]]. + * Symmetry: Are the [[AP_Statistics_Curriculum_2007_EDA_Center |mean and median]] of the dataset equal (Mean = Median)? Use the [[AP_Statistics_Curriculum_2007_Distrib_MeanVar#Notable_Moments | skewness measure]]. - * Flatness: Is the data distribution as flat as the Normal distribution? Use the [[AP_Statistics_Curriculum_2007_Distrib_MeanVar |kurtosis measure]]. + * Flatness: Is the data distribution as flat as the Normal distribution? Use the [[AP_Statistics_Curriculum_2007_Distrib_MeanVar#Notable_Moments |kurtosis measure]]. * Do [[SOCR_EduMaterials_Activities_Histogram_Graphs | histogram]], [[SOCR_EduMaterials_Activities_BoxPlot | box-and-whisker]] or [[SOCR_EduMaterials_Activities_DotChart |dotplot]] and look for bias (skewness), asymmetry, outliers, etc. * Do [[SOCR_EduMaterials_Activities_Histogram_Graphs | histogram]], [[SOCR_EduMaterials_Activities_BoxPlot | box-and-whisker]] or [[SOCR_EduMaterials_Activities_DotChart |dotplot]] and look for bias (skewness), asymmetry, outliers, etc. * Empirical Rule - check the percent of data that falls within 1, 2 and 3 [[AP_Statistics_Curriculum_2007_EDA_Var | SD]]s from the mean (should be approximately 68%, 95% and 99.7%). * Empirical Rule - check the percent of data that falls within 1, 2 and 3 [[AP_Statistics_Curriculum_2007_EDA_Var | SD]]s from the mean (should be approximately 68%, 95% and 99.7%).

## General Advance-Placement (AP) Statistics Curriculum - Non-Standard Normal Distribution and Experiments: Finding Probabilities

Due to the Central Limit Theorem, the Normal Distribution is perhaps the most important model for studying various quantitative phenomena. Many numerical measurements (e.g., weight, time, etc.) can be well approximated by the normal distribution. While the mechanisms underlying natural processes may often be unknown, the use of the normal model can be theoretically justified by assuming that many small, independent effects are additively contributing to each observation.

### General Normal Distribution

The (general) Normal Distribution, N(μ,σ2), is a continuous distribution that has similar exact areas, bound in terms of its mean, like the Standard Normal Distribution and the x-axis on the symmetric intervals around the origin:

• The area: μ − σ < x < μ + σ = 0.8413 − 0.1587 = 0.6826
• The area: μ − 2σ < x < μ + 2σ = 0.9772 − 0.0228 = 0.9544
• The area: μ − 3σ < x < μ + 3σ = 0.9987 − 0.0013 = 0.9974
• Note that the inflection points (f''(x) = 0)of the (general) Normal density function are $\pm \sigma$.
• General Normal Density Function $f(x)= {e^{{-(x-\mu)^2} \over 2\sigma^2} \over \sqrt{2 \pi\sigma^2}}.$
• The relation between the Standard and the General Normal Distribution is provided by these simple linear transformations (Suppose X denotes General and Z denotes Standard Normal Random Variables):
$Z = {X-\mu \over \sigma}$ converts general normal scores to standard (Z) values.
$X = \mu +Z\times\sigma$ converts standard scores to general normal values.

### Examples

#### Systolic Arterial Pressure Example

Suppose that the average systolic blood pressure (SBP) for a Los Angeles freeway commuter follows a Normal distribution with mean 130 mmHg and standard deviation 20 mmHg. Denote X to be the random variable representing the SBP measure for a randomly chosen commuter. Then $X\sim N(\mu=130, \sigma^2 =20^2)$.

• Find the percentage of LA freeway commuters that have a SBP less than 100. That is compute the following probability: p=P(X<100)=? (p=0.066776)
• If normal SBP is defined by the range [110 ; 140], and we take a random sample of 1,000 commuters and measure their SBP, how many would be expected to have normal SBP? (Number = 1,000P(110<X<140)= 1,000*0.532807=532.807).
• What is the 90th percentile for the SBP? That is what is xo, so that P(X < xo) = 0.9?
• What is the range of SBP values that contain the central 80% of the SBPs for all commuters? That is what are xo,x1, so that P(x0 < X < x1) = 0.8 and ${x_o+x_1\over2}=\mu=130$ (i.e., they are symmetric around the mean)? (xo = 104,x1 = 156)

### Assessing Normality

How can we tell if data collected from a process or experiment we observe is normally distributed? There are several methods for checking normality:

• Why do we care if the data is normally distributed? Having evidence that the data we are analyzing is normally distributed allows us to use the (General) Normal distribution as a model to calculate the probabilities of various events and assess significant observations.
• Example: Suppose we are given the heights for 11 women.
• First we need to show that there is no evidence suggesting that the Normal and Data distributions are significantly distinct.
• Then, we want to use the normal distribution to make inference on women heights. If the height of a randomly chosen woman is measured, how likely is that she'll be taller than 60 inches? 70 inches? Between 55 and 65 inches?
 Height (in.) 61 62.5 63 64 64.5 65 66.5 67 68 68.5 70.5