1nd Edition CJUS K300 Lecture 10 Outline of Last Lecture I Confidence Intervals pt 1 Large Samples II Central Limit Theorem Outline of Current Lecture III IV Confidence Intervals pt 2 Small Samples Confidence Intervals for Percents of Large Samples Current Lecture Student s t distribution A collection of distributions whose shape depends on degrees of freedom df n 1 Confidence intervals with small samples n 50 x t a s n 1 Steps 1 2 3 4 Determine your alpha level Calculate your degrees of freedom n 1 Use a t table to look up the t value for that alpha level and degrees of freedom Calculate your confidence interval In a sample of 20 men arrested for domestic assault the mean sentence length was 40 days with a standard deviation of 14 days Calculate a 95 confidence interval Alpha 05 df 19 talpha 2 093 40 2 093 14 10 1 40 6 72 40 6 72 46 72 40 6 72 33 28 Interpretation We are 95 confident that in the population of men arrested for domestic assault the mean sentence is between 33 28 46 72 days MAKE SURE YOU KNOW HOW TO WRITE THIS OUT FOR EVERY PROBLEM almost more important than the numbers because it shows you know what the answer actually means These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Confidence Intervals with Percents using Large Samples p z a p 1 p n P population percent p sample percent Steps 1 Determine your alpha level 2 Use the short form of the z table to look up the z crit associated with that alpha level 3 Calculate your confidence interval In an exit poll n 2000 John Brown received 68 of the vote Calculate a 95 confidence interval 68 1 96 68 1 68 2000 68 02 We are 95 confident that in the population of voters John Brown reveived between 66 and 70 of the vote PERCENTS NOT MEANS
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