Statistics Chapter 2 Exploring Distributions

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# Statistics Chapter 2 Exploring Distributions - PowerPoint PPT Presentation

Section 2.5 The Normal Distribution. Statistics Chapter 2 Exploring Distributions. Central Intervals for Normal Dist. 68% of values lie within 1 SD of the mean. Including to the right and left 90% of the values lie with 1.645 SDs of the mean.

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Section 2.5

The Normal Distribution

### StatisticsChapter 2 Exploring Distributions

Central Intervals for Normal Dist.
• 68% of values lie within 1 SD of the mean.
• Including to the right and left
• 90% of the values lie with 1.645 SDs of the mean.
• 95% lie within about 2 SDs (actually 1.96 SDs) of the mean.
• 99.7% of the data lie within 3 SDs of the mean.
Why do we study the Normal Distribution?
• Very common distribution of data throughout many disciplines.
• SAT / ACT scores
• Measure of diameter of tennis balls
• Heights / weights of people
• Once we know a distribution is Normal, there is a tremendous amount of information we can determine or predict about it.
Normal Distribution
• All normal distributions have the same basic shape.
• The difference: tall and thin vs. short and fat
• However, we could easily stretch the scale of the tall thin curve to make it identical to the short fat one.
• The area under the curve can be thought of in terms of proportions or percentage of data.
• The total area under the curve is 1.0 (100%)
The Standard Normal Distribution
• We can standardize any normal curve to be identical.
• We do this by treating the mean as Zero and the SD as One.
• The variable along the x-axis becomes what we call a z score.
• The z score is the number of SDs away from the mean.
Finding z scores for the Standard Normal Distribution
• Practice problems:
• 1) Normal distribution with:
• mean = 45 and SD = 5
• Find the z score for a data value of 19
• Find the z score for a data value of 52
• 2) Normal distribution with:
• Mean = 212 and SD = 24
• Find the z score for a data value of 236
Proportion of data in a range
• We can use the standard normal curve to find proportion of data in a range of values.
• Normal Curve example: SAT I Math scores
• Mean = 500 SD = 40
• Find the proportion of data in the score range 575 or less.
• Using z tables: Table A very back of book
• Find the proportion of data above 575.
• Find the proportion of data between 490 and 550.
Homework
• Be prepared for quiz on Tuesday.
2.5 Quiz
• You have collected data regarding the weights of boys in a local middle school. The distribution is roughly normal. The mean is 113 lbs and the SD is 10 lbs.
• A) What proportion of boys are below 100 lbs?
• B) What proportion are above 120 lbs?
• C) What proportion are in between 90 & 120 lbs?
Using calculator for proportions
• You can also use the TI-83 or higher to find these same proportions:
• 2nd , Distr, normalcdf(low, high,mean,SD)
• When using z scores you can leave mean,SD blank. normalcdf(low, high) it will default to mean=0 and SD=1.
• This will give you the same area under the curve (proportion of data) as the z table.
Finding the z score from the Percent
• If you know the percent of data covered under a normal distribution, you can find the z-score.
• Simply look up the percent (proportion) in the z table and relate it to the corresponding z score.
• Find the value that is closest to the percent given
• Another method is with the calculator.
• 2nd ,Distr, invNorm(proportion, mean, SD)
Example
• Find the z-score that has the given percent of values below it in a standard normal distribution:
• a) 32% b) 41% (use the z-table)
• c) 87% d) 94% (use your calculator)
Using the z-score to find a value
• If you know how many SDs a value is from the mean, you can use this (z-score) to find the actual data value:
• x = mean + (z • SD)
• Example: The mean weight of the boys at a middle school is 113 lbs, with a SD of 10 lbs. One boy is determined to be 2.2 SDs above the mean. How much did the boy weigh?
Combining the last two situations
• So now, if you know the percentage of data above or below a data value and you know the mean and SD, you can figure out that data value:
• Use z-table to find the z-score, then use the z score with mean and SD to find the data value.
• Or you can use the invNorm function on your calc.
• invNorm(proportion, mean, SD)
Example
• The heights of U.S. 18-24 yr old females is roughly normally distributed with a mean of 64.8 in. and a SD of 2.5 in.
• Estimate the percent of women above 5’8”
• What height would a US female be if she was 1.5 SDs below the mean? Give your answer in ft & in.
• What height would a US female be if she was considered to be in the 80th percentile?
Review Examples
• What percentage of US females is above 5’7”?
• What percent are between 5’7” and 5’0”?
Trick Question
• The cars in Clunkerville have a mean age of 12 years and a SD of 8 years. What percentage of cars are more than 4 years old?
• Why is this a trick question?
Homework
• Page 93
• E59, 61, 63, 64, 67, 69, 71, 73, 74