STAT1412 Data Analysis Laboratory
Assignment 2
Semester 2 2014
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Due:
• This assignment must be submitted electronically using Assignment 2 Link on FLO
provided under Week 9 by 12pm (noon) of Friday 10th October.
• The link will be opened until Thursday 16 October 12pm (noon) for late
submission (late penalty applies as mentioned in the SAM).
• Hard copy submission or submission by email will not be accepted.
Weighting: This assignment (out of 35 marks) comprises a total of 3 questions and is worth
10% of your final assessment mark.

Instructions:
• You MUST comply to Academic Integrity as indicated on the electronic submission.
Please note that this is an INDIVIDUAL assignment, not a group assignment.
Inappropriate collaboration will be penalized.
• Your submission should contain one (1) file in PDF format with size no bigger than 20
MB.
• You can update your submission for unlimited number of times before the due date.
• Refer to the “Statement of Assessment” pdf document on FLO regarding late assignment
penalties.
• Medical extension or extension due to compassionate ground may be granted. Only
applications with legitimate reasons will be considered.
• Keep a copy of the submission yourself.
Writing Up Your Assignment....
• Answer all questions in this assignment. Questions should be answered in the order they
appear.
• MS-Word (or other typesetting software of your choice) may be used in preparing your
assignment submission whenever appropriate which would then being converted to pdf.
• You may use any of the tools that you have been shown to assist with calculations.
Answers must be written in clear English sentences with all appropriate working and/or
supporting computer output shown. Raw computer output without explanatory text is
unacceptable.
• All workings and intermediate answers must be clearly shown.
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Question 1 [Total: 12 marks]
Electric and magnetic fields (EMF) are part of the natural environment and are present in the
earth’s core and atmosphere. These fields are also produced wherever electricity or electrical
equipment is used. These fields come from the wires that transport electricity to our homes as
Page 1 of 3well as all devices which use electricity in the home. Exposure to EMF has, in some studies,
been linked with human health. One such link has been made with brain cancer.
The recommended magnetic field exposure limit over a 24 hour period is 1000 milligauss.
Exposure to levels in excess of 1000 milligauss is considered as high exposure.
Information was collected from a sample of 200 male adult subjects.
Of the total sample, 20 subjects were exposed to high levels of EMF.
Of the total sample, 10 subjects had a positive diagnosis of brain cancer.
Of those exposed to high levels of EMF, 30% had a positive diagnosis of brain cancer.
What is the probability of the following events? Hint: A cross tabulation can help.
(a) A randomly selected male adult does not have brain cancer. [1 mark]
(b) A randomly selected male adult has a positive diagnosis of brain cancer given that he is
exposed to normal levels of EMF. [3 marks]
(c) A randomly selected adult male tests negative for brain cancer given that he is exposed to
high levels of EMF. [3 marks]
(d) A randomly selected male adult who has brain cancer and is exposed to high levels of
EMF? [2 marks]
(e) Are the events “exposed to high levels of EMF” and “have brain cancer” independent?
Show your working. [3 marks]
Marking Criteria: You need to show all working. No marks awarded for the correct answer
without working out. For full marks, you need to demonstrate understanding of the
probability concepts involved in each part of this question as well as an answer expressed in
an English sentence.
Question 2 [Total: 10 marks]
Typographic errors in a text are either non word errors (as when “the” is typed as “teh”) or
word errors that result in a real but incorrect word. Spell-checking software will catch non
word errors but not word errors. Human proof readers catch 70% of word errors. You ask a
fellow student to proofread an essay in which you have deliberately made 20 word errors.
(a) If the student matches the usual 70% rate, what is the distribution of the number of errors
missed? Explain your reasoning. [4 marks]
(b) Missing 9 or more out of 20 errors seems to be an indicator for a poor performance. What
is the probability that a proof reader who catches 70% of word errors misses 9 or more
out of 20? [3 marks]
(c) What is the probability that a proof reader who catches 70% of word errors misses at most
five word errors? [3 marks]
Marking Criteria: You may use any of the tools that you have been shown to determine the
necessary probabilities but simply cutting and pasting from the tools is not an adequate
answer. For full marks, you need to demonstrate understanding of the probability concepts
involved in each part of this question as well as an answer expressed in an English sentence.
Page 2 of 3Question 3 [Total: 13 marks]
Social networking sites, for example Facebook, MySpace and Black Planet, have grown in
popularity as users create web pages loaded with music, photographs, and profiles. Hitwise
reported that the mean time spent by a user at Facebook during April 2010 was 20 minutes.
Suppose the distribution of time spent at Facebook per month is normally distributed, with a
µ = 20 minutes and the s = 4.5 minutes. If a Facebook user is selected at random:
(a) Find the probability that the user spends less than 16 minutes per month at the site.
[2 marks]
(b) Find the probability that the user spends between 20 and 30 minutes per month at the site.
[3 marks]
(c) What is the amount of time per month a user spends on Facebook, if only 1% of users
spend this time or longer on Facebook? [4 marks]
(d) Between what values do the time spent of the middle 90% distribution of Facebook users
fall? [4 marks]
Marking Criteria: You may use any of the tools that you have been shown to determine the
necessary probabilities but cutting and pasting from the tools is not an adequate answer. For
full marks, you need to demonstrate understanding of the probability concepts involved in
each part of this question. Calculation of relevant Z scores is expected as well as an answer
expressed in an English sentence.
End of Assignment 2
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