Princeton University
Department of Operations Research
and Financial Engineering
ORF 245 – Fundamentals of Engineering Statistics
Midterm Exam 1
March 06, 2009
10:00am-10:50am
PLEASE DO NOT TURN THIS PAGE AND START THE EXAM
UNTIL YOU ARE TOLD TO DO SO.
Instructions: This exam is open book and open notes. Calculators are
allowed, but not computers or the use of statistical software packages. Write
all your work in the space provided after each question. Use the other side of
the page, if necessary. Explain as thoroughly and as clearly as possible all
your steps in answering each question. Full or partial credit can only be
granted if intermediate steps are clearly indicated.
Name: _______________________________________________________
Pledge: I pledge my honor that I have not violated the honor code during
this examination.
Pledge: ______________________________________________________
_____________________________________________________________
Signature: ____________________________________________________
1: (20) _________________
3: (20) _________________
2: (30) _________________
4: (30) _________________
Total: (100) _____________
1. (20 pts.) A company divides its production of 4-foot by 8-food woodgrained panels into three different quality groups (I, II, and III). Samples
of ten sets of 100 panels each were randomly taken from each group and
the number of imperfections in each set was counted. The box-plot
displays of the imperfection counts in each group sample are given in the
following figure (the numbers printed by the box-plots are the
corresponding readings in the vertical axis – they were added for
convenience only).
a) What was the median number of imperfections observed for each
one of the group samples?
b) What was the inter quartile range (IQR) for each group?
c) What were the minimum and maximum observed values in each
group? Are there any outliers? If so, what are their values?
d) What can you conclude about the quality of each group, as
measured by the number of imperfections found? Comment in
terms of both measures: centeredness and variability of the data.
2. (30 pts.) Consider the following probability questions. (Note that correct
answers expressed in terms of factorials or combinations are acceptable.)
a) Four engineers, A, B, C, and D, have been scheduled for job
interviews at 10 am on Friday, Jan 13, at Random Sampling, Inc.
The personnel manager has scheduled the four for interview rooms
1, 2, 3, and 4, respectively. However, the manager’s secretary does
not know this, and as a result assigns them to the four rooms in a
completely random fashion (what else!). What is the probability
that none of the four ends up in the correct room?
b) A bowl contains 20 chips, of which 9 are red, 8 are white, and 3
are blue. Six chips are taken at random and without replacement.
What is the probability that there is at least 1 white and at least 1
blue chip among the 6 selected?
c) A lot of 100 fuses is accepted if at least 9 of the 10 fuses taken at
random “blow” at the correct amperage. If there are exactly 20
defectives among the 100 fuses (that is, 20 that will blow at the
wrong amperage), what is the probability of accepting the lot?
3. (20 pts.) Box A1 contains three cubes and two balls, and box A2 contains
two cubes and five balls. A fair die is cast. If the outcome is “five or six,”
one object is taken from A1 ; otherwise an object is taken from A2 .
a) Compute the probability that a ball (say, B ) is taken.
b) Given that a ball is observed, compute the conditional (posterior)
probability that it came from A1 .
c) Compare P ( A1 | B ) determined in part (b) with the prior
probability P( A1 ) . Does the result agree with your intuition?
4. (30 pts.) A certain market has both an express and a superexpress
checkout line. Let X 1 denote the number of customers in line at the
express checkout at a particular time of the day, and let X 2 denote the
number of customers in line at the superexpress checkout at the same
time. Suppose the joint p.m.f. of X 1 and X 2 is as given in the
accompanying table.
x2
x1
0
1
2
3
4
0
.08
.06
.05
.00
.00
1
.07
.15
.04
.03
.01
2
.04
.05
.10
.04
.05
3
.00
.04
.06
.07
.06
a) What is P ( X 1 = X 2 ) , that is, the probability that the number of
customers in the two lines are identical?
b) Let A denote the event that there are at least two more customers
in one line than in the other line. Express A in terms of X 1 and X 2 ,
and calculate the probability of this event.
c) Determine the marginal p.m.f. of X 1 .
d) Calculate the expected number of customers in line at the express
checkout.
e) Find the variance of the number of customers in line at the express
checkout.
f) By inspection of the probabilities P ( X 1 = 4) , P ( X 2 = 0) , and
P ( X 1 = 4, X 2 = 0) , are X 1 and X 2 independent random variables?
Explain.