1. In August of 2020, D. R. Boulware and colleagues published a research
report in the New England Journal of Medicine. The scientists had studied
~800 volunteers who had no symptoms of illness, but who had recently been
exposed, at home or at work, to someone infected with SARS-CoV-2, the virus
that causes Covid-19. They assigned the volunteers at random to two groups.
One group received pills containing the drug hydroxychloroquine; the other
group got a placebo (pills containing no medicine). Two weeks later, the
scientists tested the volunteers to see if they were infected with the virus. The
graph summarizes the results. The black lines represent ± 1 standard error.
Instructor's answers:
a. In ONE sentence, what is the hypothesis the scientists sought to test with their experiment?
In people recently exposed to SARS-CoV-2, hydroxychloroquine reduces the risk of developing an
active viral infection. [A full-credit answer would limit the scope of the hypothesis to people like the
volunteers in the study. A partial credit answer might simply say that the drug reduces the risk of
developing Covid-19.]
b. In ONE sentence, what is the null hypothesis?
Hydroxychloroquine has no effect on the risk that a recently exposed person will develop Covid-19. [A
full credit answer would be more specific than "the drug has no effect."]
c. In ONE sentence, what does a comparison of the heights of the two colored bars indicate?
The rate of Covid-19 was somewhat higher among volunteers who got the placebo than in the
volunteers who got hydroxychloroquine. [A full-credit answer would acknowledge what the bars
themselves do show a difference—but it would not over-interpret that difference.]
d. In ONE sentence, what does a comparison of the standard error bars indicate?
The standard error bars overlap which suggests, as a rule of thumb, that there is not sufficient
evidence to reject the null hypothesis that the rate of infection is the same in both groups. [A fullcredit answer would recognize that the standard error bars give reason to treat the difference between
groups with skepticism. As it is based on a rule of thumb, however, it would not make definitive claims.]
e. The scientists report a p-value of 0.35. In ONE sentence, what does this mean?
Assuming the null hypothesis is true, there is a 35% chance that an experiment like this one would
yield a difference between groups at least as large as the one that appears in the graph. [A full credit
answer will indicate that the p-value is calculated under the assumption that the null hypothesis is
true. A full credit answer will also indicate that the p-value is the probability of getting a difference
between groups as large or larger than the one we see in the graph. (The p-value does NOT give us the
probability that the null hypothesis is true or false. The p-value does NOT give us the probability that the
alternative hypothesis is true or false. The p-value does NOT indicate that there is a 35% probability that
the difference we see in the graph is due to chance.)]
f. In ONE sentence, what conclusions can we draw from the data and the analysis?
The data don't provide sufficient reason to reject the null hypothesis. [A full-credit answer will indicate
that we should maintain our skepticism about the benefits of hydroxychloroquine. However, it should
not make definitive statements about the alternative hypothesis. The data don't rule out the possibility
that the drug provides a modest benefit.]
g. Keep in mind that hydroxycholorquine can cause side effects and that research likes this takes money
and time. In TWO or THREE sentences, state whether you would / would not support a replication of this
experiment, and justify your stance.
Many answers are possible. For example, "Even if the protection offered by hydroxychloroquine is
small, if used at large scale the drug could save lives. I therefore support further research to establish
the size of the benefit—if there is one." OR "This study provides little reason to think that
hydroxychloroquine protects recently exposed people from developing Covid-19. I therefore feel that
our research effort should be directed elsewhere." [You will not be graded on whether you support
further research. You will be graded on whether your reasons are sound and support your position.]
2. The diagram above shows the design of an experiment by education
researcher Nate Kornell. Across four consecutive days, student
volunteers studied a total of 40 flash cards, each with a pair of words
that were synonyms (such as effulgent : brilliant). On Day1, the
students looked at cards 1-20 twice each, and cards 21- 25 eight times
each. On Day 2, the students looked at cards 1-20 twice each, and
cards 26-30 eight times each, and so on. Kornell refers to studying a
card lightly each day "Spaced" studying; studying a card intensely on a
single day is "Massed" studying. On the fifth day, the students took a
test covering all 40 cards. Their average performance on the different
groups of cards is graphed on the right. Address the following:
a. In ONE sentence, what is the pattern in the data as regards massed versus spaced studying?
On average, the students earned better exam scores on the cards they studied in the Spaced pattern
than they did on the cards they studied in the Massed pattern. [A full credit answer would concisely
make this one comparison, and acknowledge that this conclusion is based on the average performance
of the students on the different sets of cards.]
b. Many students wait until the night before an exam, when they study intensely. Which TWO bars
represent the best analogy to a comparison between cramming the night before an exam versus
studying a little bit each day during the week before the exam?
Massed Session 4, representing cramming the night before, versus Spaced, representing studying a
little bit each day. [A full credit answer would pick the right two bars and identify which bar represents
which study strategy.]
c. In ONE sentence, what is the take-home message for a student wanting to do well on exams in Biol
180?
Kornell's study suggests that cramming on Thursday works better than cramming on Tuesday, but—all
else being equal—studying a little bit every day on Monday, Tuesday, Wednesday, and Thursday is the
best strategy. [A full credit answer would draw the conclusion that spaced studying is the best strategy,
and acknowledge that this experiment is not a perfect analogy to preparing for a Biology 180 exam.]
3. Hormones called angiogenesis inhibitors (here, AI's) slow the rate at which blood vessels grow into
tissues. A tumor is a growing lump of tissue. It cannot grow beyond a certain size unless blood vessels
grow into it. This suggests the hypothesis that AI's might be useful anti- cancer drugs. Design an
experiment to test this hypothesis. You have laboratory mice into which you can implant tumors, a
supply of AI's, and an unlimited supply of cages, water, and mouse chow.
Instructor's answers:
a. What is your independent variable?
Whether AI is given to the experimental mice. [This could also be the amount of AI given to the mice.]
b. What are your treatment groups?
Several dozen mice per group, implanted with tumors, then injected with a dose of AI or a dose of
drug-free saltwater. [This could also be mice implanted with tumors, then injected with, say, a small,
medium, or large dose of AI.]
c. What is your dependent (response) variable?
Size of the tumor 1 month after implantation and treatment. [There other possibilities, such as how
long the mice survive.]
d. Name another variable you will control. In ONE sentence, explain how.
There are many possibilities: Sex of the mice, controlled by making sure each group is half females and
half males; Emotional stress experienced by the mice, controlled by making sure all mice are kept in
clean cages, with companionship, space, a stimulating environment, and the opportunity to exercise;
Age of the mice, controlled by enrolling them all when they are, say, six weeks old; etc.
e. On scratch paper, draw graphs predicting the results of your experiment if (i) the data support your
hypothesis, and (ii) if the data cannot reject the null hypothesis. In the Canvas textbox, describe both
graphs in words (do not attempt to paste or upload the actual graphs).
If the data fail to reject the null hypothesis, the mean tumor size would not be different between the
two treatment groups (with and without AI) – the bars on the graph will be the same height. On the
other hand, if the data reject the null hypothesis, the size of the tumor will be different between the
two treatments: if AI is an effective cancer drug, the tumors in mice injected with AI will be smaller
than the tumors in mice injected with saltwater. [You will not be graded for your actual figures, but
rather, for the description of your figures.]
f. Imagine that you are on a University Panel evaluating the ethics of the experiment you have just
described. In TWO or THREE sentences, state whether you would approve this research and give your
reasons.
This is a question on which reasonable people can disagree. Possible answers include: "As long as the
mice are treated with respect and well cared-for, I think the experiment is justified. It could lead to
cancer treatments that alleviate human suffering." or "I don't feel that this experiment is ethical. I
don't believe it is acceptable to use sentient creatures for our own purposes." [You will not be graded
on whether you feel the research is ethical. You will be graded on whether your reasons are sound and
support your position.]