1) The first task is to review some information that might be
useful later:
a) Write a brief definition of the word "quartile" as we have
used it in previous weeks. Be sure to provide a citation:
_____________________________.
b) Write a brief definition of the word "quantile" as it might
be used in statistics. Be sure to provide a citation (do not cut
and paste... use your own words to summarize what you discovered):
________________________________.
c) From within interactive R, enter the command shown below
(the command shows a help page for the pbinom command). Provide a
very brief description of the arguments that are passed to the
pbinom() command ("arguments" in computer programming are the
options that you give to a function so that the function can
calculate what you want it to). Note that one of the arguments is
lower.tail = TRUE, and because there is a value assigned to it with
the equals sign, it means that if you do not enter a new value for
lower.tail, it will be set to TRUE by default. Do not type the
">" into R, it is the command prompt:
> ?pbinom
2) You can use the dbinom() command (function) in R to
determine the probability of getting 0 heads when you flip a fair
coin four times (the probability of getting heads is 0.5):
dbinom(0, size=4, prob=0.5)
Find the equivalent values for getting 1, 2, 3, or 4 heads
when you flip the coin four times. TIP: after you run the first
dbinom() command, press the up arrow and make a small change and
run it again.
probability of getting exactly 1 head: _______
probability of getting exactly 2 heads: _______
probability of getting exactly 3 heads: _______
probability of getting exactly 4 heads: _______
3) Use the pbinom() function in R to show the cumulative
probability of getting 0, 1, 2, 3, or 4 heads when you flip the
coin 4 times (this is the same as finding the probability than the
value is less than or equal to 0, 1, 2, 3, or 4.)
probability of getting no more than 0 heads: ____
probability of getting no more than 1 head: _____
probability of getting no more than 2 heads:_____
probability of getting no more than 3 heads: ____
probability of getting no more than 4 heads: ____
4) The following R command will show the probability of
exactly 6 successes in an experiment that has 10 trials in which
the probability of success for each trial is 0.5:
dbinom(6, size=10, prob=0.5)
(True/False)____________
5) What is the probability of getting fewer than 2 heads when
you flip a fair coin 3 times (round to 2 decimal places) ?
______
6) What is the probability of getting no more than 3 heads
when you flip a fair coin 5 times (be sure to understand the
wording differences between this question and the previous
one—round to 2 decimal places)? ________
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Information
The exponential distribution is a continuous distribution. The
main R functions that we will use for the exponential distribution
are pexp() and qexp(). Here is an example of the pexp() function.
Leaves are falling from a tree at a rate of 10 leaves per minute.
The rate is 10, or we can say that lambda = 10 (meaning 10 leaves
fall per minute). The leaves do not fall like clockwork, so the
time between leaves falling varies. If the time between leaves
falling can be modeled with an exponential distribution, then the
probability that the time between leaves falling will be less than
5 seconds (which is 5/60 of a minute) would be:
(note: this is an explanation of how pexp() works, you will
answer a different question below)
pexp(5/60, rate=10)
which is about 0.565 (meaning that the probability is a bit
higher than 50% that the next time-span between leaves falling will
be less than 5 seconds).
For tasks 7-12, assume that the time interval between
customers entering your store can be modeled using an exponential
distribution. You know that you have an average of 4 customers per
minute, so the rate is 4, or you can say that lambda =4.
It is easiest to keep everything in the original units of
measurement (minutes), but you can also translate that to seconds
because a rate of “4 customers per minute” is the same as “4
customer per 60 seconds,” and you can divide each number by 4 to
get a rate of “1 customer per 15 seconds” or a rate of “1/15
customers per second.”
7) What is the expectation for the time interval between
customers entering the store? Be sure to specify the units of
measurement in your answer. Round to 3 decimal places:
___________________
8) What is the variance of the the time interval? Be sure to
specify the units of measurement in your answer. Round to 3 decimal
places:_________________
9) The pexp() function is introduced at the bottom of Yakir,
2011, p. 79, and there are some tips above. What is the probability
that the time interval between customers entering the store will be
less than 15.5 seconds. Be sure to enter values so that everything
is in the same unit of measurement. Be sure to specify the units of
measurement in your answer. Round your answer to 3 decimal places:
_________________.
10) What is the probability that the time interval between
customers entering the store will be between 10.7 seconds and 40.2
seconds?________
11) The qexp() function in R allows you to enter a
probability, and it will tell you the criterion value (“cutoff
point”) that corresponds to that probability value (e.g., if you
enter .05 it tells you the cutoff point below which 5% of the
values in the distribution fall).
What value of x would be the criterion value (cut-off point)
for the top 5% of time intervals (Round to 3 decimal places, and
include the units of measurement)? _______
---------------------------------
12) Describe in your own words the meaning of the number that
the following R command produces (you are asked to interpret the
resulting number so that we understand what that number
means).
pexp(1.2, rate=3)
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Information
You have now had practice with the binomial distribution and
the exponential distribution. The approach to solving problems for
the normal distribution is similar to that for the exponential
function, but obviously you use different R functions (usually
pnorm() or qnorm()).
For the following three exercises, assume that you have a
normally distributed random variable, called A, with a mean of 7,
and a population standard deviation of 3.
13) What is the probability that a randomly selected value
from variable A will be greater than 9?_______
14) What value of variable A would be the cutoff point
(criterion value) for identifying the lowest 4% of values in
variable A (use the qnorm function)?____________
15) What is the probability that a randomly selected value
from variable A will be more than one standard deviation above its
mean (there are couple ways to solve this, one way is to use the
standard normal distribution?________________