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STATISTICS FOR SOCIAL & BEHAVIORAL SCIENCES
MID TERM 2 – November 25, 2014

This midterm is made of 7 problems on 10 numbered pages, for a total of
54 points.

You have 70 minutes (one hour and 10 minutes) to complete this midterm.

Write your name at the top of this sheet and on every page.

Follow each problem’s instructions. There is one box to tick, unless
indicated. Provide your numerical answer on the line provided in clear
handwriting.

Non-communicating calculators are recommended (no 3g, wifi, LTE, 2g,
EDGE, even in airport mode). The calculator’s memory should not contain
the course’s statistical formulas.

Please perform this mid term 2 in silence.

If stuck on one question, move on ! There are easy and tough questions
on this midterm, so keep going!

This is a closed book midterm. Notes are not allowed.

If in doubt about a question, write on the answer sheet provided and move
on. The rule is that I will not provide clarifications during the exam.

This midterm should be enjoyable: good luck !

The table of t scores is provided on page 10.
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Problem 1 – Sampling distribution of the t statistic (10 points)
A researcher on Hepatitis B has a sample of 419,221 individuals, and he would
like to test the null hypothesis that the average fraction of individuals with
Hepatitis B is equal to 3%. In his sample, 2.8% of individuals have Hepatitis B.
He takes a sheet a paper to remind himself of the sampling distribution of the t
statistic.
On the diagram below of the sampling distribution of the t statistic, draw/mark:
▪ the bell shaped sampling distribution of the t statistic
▪the label of the horizontal axis
▪the label of the vertical axis
▪the mean of the sampling distribution of the t statistic
▪the standard deviation of the sampling distribution t statistic (mark a segment)
▪the t score (and minus the t score) for confidence levels 90%, 95%, 99%.
▪the fraction of t statistics that are above the t score at 95% confidence level.
▪ the fraction of t statistics that are below minus the t score at 95% confidence
level.
The sampling distribution of the t statistic:
is right skewed
is symmetric
is left skewed
none of the above
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Problem 2 – True or False? (6 points)
An NYU Abu Dhabi student did not take “statistics for social sciences” but read
the book independently. He tends to make some mistakes.
What assertions are true? Tick the only answer that applies.
1. For a test at 95% confidence that the population mean is equal to 0.4, the
probability of Type II error is 5%.
True.
False, the probability of Type II error is 95%.
False, the probability of Type II error is unknown.
2. As sample size increases, for a test at given significance level, the probability
of Type I error decreases.
True.
False, the probability of Type I error increases, as the standard error
decreases.
False, the probability of Type I error is constant.
3. For a t test, the null hypothesis is that the sample mean is equal to a given
value (say 0.5)
True
False, the null hypothesis is that the population mean is equal to a
given value.
False, the null hypothesis is that the population mean is different from
a given value.
False, the null hypothesis is that the sample mean is different from a
given value.
4. For a large sample size, the standard deviation of the sampling distribution of
the t statistic is 1
True
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False, the standard deviation of the sampling distribution of the t
statistic is 1.96.
False, the standard deviation of the sampling distribution of the t
statistic is 2 times 1.96.
5. As the confidence level increases, the confidence interval for the sample mean
gets larger.
True
False
6. The standard error of a sample mean of X is always smaller or equal than the
standard deviation of the variable X.
True.
False, it depends on the sample.
False, is always larger.
7. The sampling distribution of the t statistic has thinner tails than the normal
distribution. In other words, the probability of extremely large values of the t
statistic is smaller in the t distribution than in the normal distribution.
True
False
8. The larger the number of degrees of freedom of a t distribution, the smaller the
t score at 95%.
True
False, the larger the t score.

False, it depends on the standard deviation of the sample.
Problem 3 – Ancient manuscript (6 points)
You discovered an old statistics manuscript written in 1966. The author (John
Enroe) said that he collected a sample by simple random sampling in the
population of Gaborone residents, with a variable “age” for each individual of the
sample.
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John Enroe in the manuscript says that he can reject the null hypothesis that the
population mean age is equal to 30 years old at 95%, but not at 99%. In his
sample, the average age was 27 years old, and the number of observations in
the sample was N=341.
The original data has been lost, and the manuscript was typed on an old
typewriter.
We would like to recover the standard deviation of age in Gaborone.
Write down the formula for the Margin of Error as a function of the sample size N,
the standard deviation of age, and the z statistic.
________________________________________________________________
The manuscript says that the null hypothesis is rejected at 95%, therefore:
The standard deviation in the sample is smaller than
The manuscript says that the null hypothesis is not rejected at 99%. Thus the
standard deviation in the sample is larger than
________________________________________________________________
Problem 4 – Bayesian doctor (9 points)
A doctor in South African has read in a study that out of 300 patients whose
medical condition had improved, 100 had taken MedPlus, while 200 patients had
not taken MedPlus. Out of 200 patients whose condition had not improved, 100
had taken Medplus, while 100 had not taken Medplus.
The overall proportion of patients whose condition has improved is 60%.
The doctor is ultimately interested in the probability that a patient improves
conditional on having taken Medplus.
1. Write down Rule #3 of probabilities for two events that are not independent:
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2. Can you qualify the event A : “Patient taking Medplus” and the event B :
“Patient improving”? Tick only the boxes that apply:
A and B are unrelated
A and B are overlapping
A and B are mutually exclusive
A and B are independent
A and B are related
3. What is the probability that a patient improves conditional on having taken
Medplus?
Problem 5 – Textual analysis (12 points)
In a library, an old series of speeches of a US president are unattributed – the
actual president who wrote those speeches is unknown. A political scientist,
Alfred Moolb, would like to find out who wrote those speeches.
In totality, those speeches have 4,000 words, and he uses the following words
with the following frequency
Word
until
whereas
task
bravery
Frequency
7%
3%
5%
0.5%
We suspect that the speeches have been authored by John Fitzgerald Kennedy.
We collect the writings of John Fitzgerald Kennedy, and find he uses these words
with the following frequency:
Word
until
whereas
task
bravery
Frequency
6%
3.4%
4%
1%
We would like to test the null hypothesis that ‘the fraction of uses of the word
“until” in the unknown president’s speeches is equal to the fraction of uses of the
word “until” for John Fitzgerald Kennedy.’ The fraction of uses of the word “until”
for John Fitzgerald Kennedy is exactly known.
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1. Give a 99% confidence interval for the fraction of uses of the word “until” in the
unknown president’s speeches:
[
,
]
2. Give the t statistic for the null hypothesis:
3. At what significance levels can we reject the null hypothesis? Tick all that
apply (points added for right answer, removed for wrong answer).
None
At 1%
At 5%
At 10%
4. Now the political scientist Alfred Moolb builds a t statistic for many null
hypothesis. Each null hypothesis is ‘the fraction of uses of the word “XXXXX” in
the unknown president’s speeches is equal to the fraction of uses of the word
“XXXXX” in John Fitzgerald Kennedy’s speeches’. He writes the null hypothesis
for 500 different words XXXXX, including until, whereas, task, bravery, and 496
other words. He builds a t statistic for each word as well.
Under the null hypothesis, how many of the 500 t statistics will be strictly greater
than 2.58 (t>2.58)?
Under the null hypothesis, how many of the 500 t statistics will be strictly lower
than -2.58 (t<-2.58)?
Under the null hypothesis, how many of the 500 t statistics will be strictly lower
than -1.65 (t<-1.65)?
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Problem 6 – Pakistan Travel (5 points)
Anthony is a research assistant for the Clinton Foundation in Pakistan. He has
collected observations on samples in different cities. In each city, he tests a null
hypothesis for the mean of a variable. The variable has a normal distribution.
Each line of this table corresponds to a separate sample of size N, and a
separate null hypothesis. Tick all boxes that apply in the last three columns.
Sample
Lahore
Islamabad
Sukkur
Larkana
Peshawar
Hyderabad
Karachi
t statistic
3.2
-1.02
4.5
-6.4
-7.8
2.1
-2.5
sample
size N
23
11
2
2
120
31
29
Reject H0
at 90% ?







8
Reject H0
at 95% ?







Reject H0
at 99% ?






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Problem 7 – Correlation and Slope
The World Bank argues that a higher fraction of women with a college degree
causes a lower rate of violence.
They found that the average fraction of women with a college degree across
countries is 25%. They also found that the standard deviation of violent acts is
10. In the same report, the correlation between the fraction of women with a
college degree and violent acts is -0.4.
Give the formula relating slope and correlation.
What is the slope of the relationship between the fraction of women with a
college degree (explanatory variable) and violence (dependent variable)?
________________________________________________________________
Thus…
 he higher the fraction of women with a college degree, the higher the
number of violent acts in a country.
 he higher the fraction of women with a college degree, the lower the
number of violent acts in a country.
…
A 1 percentage point increase in the fraction of women with a college degree
leads to a ……….. (increase/decrease) in the number of violent acts.

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Table of t Scores
END OF MIDTERM
Thank you for your answers.
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DRAFT – WRITE HERE – ONLY ANSWERS ON THE ANSWER FORM ITSELF
WILL BE CONSIDERED (FIRST 9 PAGES)
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DRAFT – WRITE HERE – ONLY ANSWERS ON THE ANSWER FORM ITSELF
WILL BE CONSIDERED (FIRST 9 PAGES)
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DRAFT – WRITE HERE – ONLY ANSWERS ON THE ANSWER FORM ITSELF
WILL BE CONSIDERED (FIRST 9 PAGES)
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