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Confidence Intervals with Means
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What is the purpose of a confidence interval?
To estimate an unknown population parameter
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Formula: Margin of error Standard deviation of statistic
Critical value statistic Margin of error
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In a randomized comparative experiment on the effects of calcium on blood pressure, researchers divided 54 healthy, white males at random into two groups, taking calcium or placebo. The paper reports a mean seated systolic blood pressure of with standard deviation of 9.3 for the placebo group. Assume systolic blood pressure is normally distributed. Can you find a z-interval for this problem? Why or why not?
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Student’s t- distribution
Developed by William Gosset Continuous distribution Unimodal, symmetrical, bell-shaped density curve Above the horizontal axis Area under the curve equals 1 Based on degrees of freedom df = n - 1
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t- curves vs standard normal curve
Graph examples of t- curves vs standard normal curve Y1: normalpdf(x) Y2: tpdf(x,2) Y3:tpdf(x,5) use the -0 Change Y3:tpdf(x,30) Window: x = [-4,4] scl =1 Y=[0,.5] scl =1
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How does the t-distributions compare to the standard normal distribution?
Shorter & more spread out More area under the tails As n increases, t-distributions become more like a standard normal distribution
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Standard error – when you substitute s for s.
Formula: Standard deviation of statistic Standard error – when you substitute s for s. Critical value statistic Margin of error
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How to find t* Use Table B for t distributions
Can also use invT on the calculator! Need upper t* value with 5% is above – so 95% is below invT(p,df) Use Table B for t distributions Look up confidence level at bottom & df on the sides df = n – 1 Find these t* 90% confidence when n = 5 95% confidence when n = 15 t* = 2.132 t* = 2.145
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Steps for doing a confidence interval:
Assumptions – Calculate the interval Write a statement about the interval in the context of the problem.
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Statement: (memorize!!)
We are ________% confident that the true mean context is between ______ and ______.
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Assumptions for t-inference
Have an SRS from population (or randomly assigned treatments) s unknown Normal (or approx. normal) distribution Given Large sample size Check graph of data Use only one of these methods to check normality
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Ex. 1) Find a 95% confidence interval for the true mean systolic blood pressure of the placebo group. Assumptions: Have randomly assigned males to treatment Systolic blood pressure is normally distributed (given). s is unknown We are 95% confident that the true mean systolic blood pressure is between and
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Ex. 2) A medical researcher measured the pulse rate of a random sample of 20 adults and found a mean pulse rate of beats per minute with a standard deviation of 3.86 beats per minute. Assume pulse rate is normally distributed. Compute a 95% confidence interval for the true mean pulse rates of adults. We are 95% confident that the true mean pulse rate of adults is between &
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Ex 2 continued) Another medical researcher claims that the true mean pulse rate for adults is 72 beats per minute. Does the evidence support or refute this? Explain. The 95% confidence interval contains the claim of 72 beats per minute. Therefore, there is no evidence to doubt the claim.
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Ex. 3) Consumer Reports tested 14 randomly selected brands of vanilla yogurt and found the following numbers of calories per serving: Compute a 98% confidence interval for the average calorie content per serving of vanilla yogurt. We are 98% confident that the true mean calorie content per serving of vanilla yogurt is between calories & calories.
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Ex 3 continued) A diet guide claims that you will get 120 calories from a serving of vanilla yogurt. What does this evidence indicate? Note: confidence intervals tell us if something is NOT EQUAL – never less or greater than! Since 120 calories is not contained within the 98% confidence interval, the evidence suggest that the average calories per serving does not equal 120 calories.
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Robust CI & p-values deal with area in the tails – is the area changed greatly when there is skewness An inference procedure is ROBUST if the confidence level or p-value doesn’t change much if the normality assumption is violated. t-procedures can be used with some skewness, as long as there are no outliers. Larger n can have more skewness. Since there is more area in the tails in t-distributions, then, if a distribution has some skewness, the tail area is not greatly affected.
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Find a sample size: If a certain margin of error is wanted, then to find the sample size necessary for that margin of error use: Always round up to the nearest person!
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Ex 4) The heights of PWSH male students is normally distributed with s = 2.5 inches. How large a sample is necessary to be accurate within inches with a 95% confidence interval? n = 43
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Some Cautions: The data MUST be a SRS from the population (or randomly assigned treatment) The formula is not correct for more complex sampling designs, i.e., stratified, etc. No way to correct for bias in data
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Cautions continued: Outliers can have a large effect on confidence interval Must know s to do a z-interval – which is unrealistic in practice
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