* Shape: When the population distributions are Normal, the
sampling distribution of 𝑥1 − 𝑥2 is Normal. In other cases, 𝑥1 −
𝑥2 will be Normal if both sample sizes are ≥ 30.
* Center: The mean of the sampling distribution is 𝜇1 − 𝜇2
* Spread: The standard deviation of the sampling distribution is
𝜎2 1
𝜎2 2
as long as each sample is no more than 10% of the
* Confidence Interval: (𝑥1 −𝑥2 ) ±
* Still have to check conditions first!
*𝑧 =
𝑥1 −𝑥2 −(𝜇1 −𝜇2 )
*𝑡 =
𝑥1 −𝑥2 −(𝜇1 −𝜇2 )
𝜎2 1 𝜎2 2
𝑠2 1 𝑠2 2
𝑛1 𝑛2
* “How do the sizes of longleaf pine trees in the northern and
southern halves of the forest compare?” To find out,
researchers took random samples of 30 trees from each half
and measured the diameter in centimeters. Comparative
boxplots of the data and summary statistics from Minitab are
found below. Construct and interpret a 90% confidence interval
for the difference in the mean diameter of longleaf pines in
the northern and southern halves of the forest.
* State:
* Plan:
* Do:
* Conclude:
* Look at pg 637 in your book…look at the formula for df
* If you use your calculator…this is what they are doing, so
your df will most likely be a decimal. If you are using
your calculator to give you your answers – you must
report what they report as df or you will get it wrong. If
you do what we normally do (n-1) then you have to keep
what the formula gives you…not what the calc. gives!
* Does increasing the amount of calcium in our diet reduce blood
pressure? The subjects were 21 healthy men who volunteered
to take part in the experiment. They were randomly assigned
to two groups: 10 of the men received a calcium supplement
for 12 weeks, while the control group of 11 men received a
placebo pill that look identical. The experiment was doubleblind. The response variable is the decrease in systolic blood
pressure for a subject after 12 weeks, in millimeters of
mercury. An increase appears as a negative response. Do the
data provide sufficient evidence to conclude that a calcium
supplement reduces blood pressure more than a placebo?
* State:
* Plan:
* Do:
* Conclude:
* The heights of ten-year-old girls follows a Normal distribution
with mean 𝜇𝑓 = 56.4 inches and standard deviation 𝜎𝑓 = 2.7
inches. The heights of ten-year-old boys follows a Normal
distribution with mean 𝜇𝑚 = 55.7 inches and standard
deviation 𝜎𝑚 = 3.8 inches. A researcher takes a random sample
of 12 ten-year-old girls and 8 ten-year-old boys in the US. After
analyzing the data, the researcher reports that the mean
height 𝑥𝑚 of the boys is larger than the mean height 𝑥𝑓 of the
girls. Describe the shape, center, and spread of the sampling
distribution 𝑥𝑓 − 𝑥𝑚 .
* Find the probability of getting a difference in
sample means 𝑥𝑓 − 𝑥𝑚 that is less than 0.
𝑥1 − 𝑥2 − (𝜇1 − 𝜇2 )
𝜎 21 𝜎 2 2
𝑛1 + 𝑛2
* Pg 652 (35, 39, 40, 45, 49, 54**, 61-66)
**For #54 – Use the “unequal” – we DO NOT pool
means problems!