Quiz 7 questions - WPMU DEV
Chapter 7 Frequently Asked Questions
Example 1
Find the area to the left of z = -1.52 and to the right of 2.52. Use the Calculator !! Remember that you will not have tables on the final exam! To the left means from negative infinity to -1.52 (we obviously can't type infinity into the calculator, so we use a very very very small number, like -10^99) normalcdf(-10^99, -1.52) = 0.0643 (rounded to four places) To the right means from 2.52 to infinity (we obviously can't type infinity into the calculator, so we use a very very very large number, like 10^99) normalcdf(2.52, 10^99) = 0.0059 (rounded to four places)
0.0643 + 0.0059 = 0.0702 (which is what they got in MSL)
Example 2
Find the z-score such that the area under the curve to the left is 0.72 invNorm gives you the z-score for the area to the left So, invNorm(.72) = 0.58 (rounded to two places)
Example 3
Find the z-scores that separate the middle 79% of the distribution from the area in the tails of the standard normal distribution. Here is a generic normal distribution:
Here is a normal dist. With 79% shaded between the tails.
So we are looking for these two z-scores: If 79% of the total area (the total area is 100%) then how much is to the LEFT of that z-score on the left? (We want to know what's on the LEFT because invNorm tells us the z-score if we know the area on the left.)
100% - 79% = 21% but that 21% is divided equally between the two tails. 21% / 2 = 10.5% So find invNorm(.105) = -1.25 (when rounded to two places)
Since these z-scores have the same area to the right and left (there is also 10.5% to the right of the other z-score) we know that it will be the mirror image on the right side. So our other z-score is +1.25 (there's no need to actually calculate it). However, just to prove that to you, let's figure out what the area to the left of that z-score would be. 79% (shaded) + 10.5% (in left tail) = 89.5% to the left invNorm(.895) = 1.25 so the z-scores are -1.25, 1.25
Example 4
Assume the random variable X is normally distributed with a mean of 50 and a standard deviation 7. Compute the probability P(34 < X < 63).
MSL will accept this answer - - 0.9572 but on some of the homework it won't match (See for more explanation.)
Example 5
The number of chocolate chips in an 18-oz bag of chocolate chip cookies is approximately normally distributed with a mean of 1252 and standard deviation 129 chips. (Round all answers to four decimal places.)
a) What is the probability that a randomly selected bag contains between 1100 and 1500 chips?
normalcdf(1100,1500,1252,129) = 0.8534 b) What is the probability that a randomly selected bag contains fewer than 1025 chips?
normalcdf(-10^99,1025, 1252,129) = 0.0392
c) What is the probability that a randomly selected bag contains more than 1200 chips?
normalcdf(1200,10^99, 1252,129) = 0.6566 However, because of the rounding issues (see for more explanation) So if you convert 1200 to a z-score as demonstrated in the above link, you could do this and get their answer:
If you ever have an answer on this quiz from the calculator that was just a few ten-thousandths off the answer in MSL, email me and you'll get full credit back. Always be sure to tell me the question number you think you have correct. d) What is the percentile rank of a bag that contains 1475 chips? The percentile is the area to the left of the z-score. Since we want the area to the left we want the 1475 value and everything below it.
normalcdf(-10^99,1475, 1252, 129) = 0.96 (rounded to two places) 0.96 is 96% so it's the 96th percentile.
Example 6
The lengths of a particular animal's pregnancies are approx. normally distributed with a mean of 279 and standard deviation 20 days. a) What proportion of pregnancies last more than 284 days? More than 284 days ? so the area to the right of 284
normalcdf(284,10^99,279,20) = 0.4013 b) What proportion of pregnancies last between 244 and 289 days?
normalcdf (244,289,279,20) = 0.6514 c) What is the probability that a randomly selected pregnancy lasts nor more than 269 days?
No more than 269 days means LESS THAN 269 days ? area to the left of 269
normalcdf (-10^99,269,279,20) = 0.3085
d) A "very preterm" baby is one whose gestation period is less than 234 days. Are very preterm babies unusual? "unusual" is if the probability is less than 0.05. So we would want to find out if the area to the left is less than 0.05. (see page 235, section 5.1)
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