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# Example constructing a t interval for a mean

Example showing how to calculate a one-sample t interval for a mean.

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• I calculated the t-score on my Ti-84 by using the invT(area, df) function, and with area=0.95 and df = 13, I got the t-score to be approx.1.761, can someone help? Thanks!
• You need to input the one-tailed area, in this case 0.975
• I used to calculate the critical point over the course of the previous videos by multiplying the value from the table by the square of p/sample n. What's new this time?
(1 vote)
• If our df is 60 and we need to do 60-1=59, which one will we choose? Still the 60 or 50?
(1 vote)
• We’ve established that heights of 10-year-old boys vary according to a Normal distribution with μ = 140 cm and σ = 5 cm.

What proportion is between 150 and 140 cm?
(1 vote)
• The weight of brains from Alzheimer cadavers varies according to a Normal distribution with mean 1077g and standard deviation 106g. The weight of an Alzheimer-free brain averages 1250 g. What proportion of brains with Alzheimer disease will weigh more than 1250 g?
(1 vote)
• Why do we divide the sample standard deviation by the square root of n? Don't the z* and t* statistic give the number of standard deviations to go away from the mean? If so, then why are we dividing something by the standard deviation? Thanks for the help!
(1 vote)
• Remember that the sample variance is equal to population variance divided by the sample size. When you square root everything you get the standard dev. You can check out the video "Standard error of the mean" to review.