First midterm study guide

This is the study guide for the first midterm in Research Methods in Biology, fall 2007. The midterm will be on Tuesday, September 25. You may not use your notes or books during the exam; if English is your second language, you may use a dictionary. You will not need to use a calculator.

Study your lecture notes and the web pages (available at the class syllabus page) on all the topics covered before the exam. Some of the web pages have been revised since they were first posted, so if you printed them out, you should go back and look at the web pages again. You should be familiar with the kinds of biological variables; the basic rules of probability; the basic concepts of statistical hypothesis testing and power analysis; and each of the statistical tests we've covered. For each statistical test, you should know when to apply the test, what statistical null hypothesis it is testing, how to interpret the results, and what the advantages and disadvantages of each test are. You do not need to memorize the equations used in the tests.

For some experiments, the choice of the best test to use depends on whether the sample sizes are "small" or "large." For the purposes of this class, a "large" sample size is greater than 1000, while "small" is less than 1000.

When a question asks for a statistical test, you must give only one answer, even if more than one would be correct (such as chi-square test of independence or G-test of independence). You must give the full name of the test; don't just say "chi-square test."

If a question uses biological words or concepts that are unfamiliar to you (for example, you don't know what a "promoter element" is), please ask me for help. You are being tested on your knowledge of methods used to analyze biological experiments, not on your knowledge of biology.

The exam will consist of about 15 to 20 short-answer questions. Most of them will be questions in which I describe a data set and you describe how to analyze it. There may also be questions in which I describe the results of a statistical analysis and you interpret the results, and questions in which I ask you to identify the types of variables (attribute, measurement or ranked) in an experiment. Here are some examples. To see the correct answer to a question, click on the "Answer" button. Don't bother typing your answer into the box, it won't do anything.

1. You are interested in the effects of fertilizer on mitosis in onion root tips. In an onion root tip grown without fertilizer, you count 70 cells in interphase, 28 cells in prophase, 2 cells in metaphase, 5 cells in anaphase, and 10 cells in telophase. In an onion root tip grown with fertilizer, you count 94 cells in interphase, 57 cells in prophase, 6 cells in metaphase, 11 cells in anaphase, and 27 cells in telophase. What statistical test would you use to analyze these data?
   What would your statistical null hypothesis be?
   What are the variables in this experiment?
2. You want to know what determines handedness in humans. Out of 216 children of women who smoked during pregnancy, 37 are left-handed. Out of 587 children of women who did not smoke during pregnancy, 43 are left-handed. You do the appropriate statistical test, and the P-value is 0.001. How would you interpret this result?
3. You want to know whether your 6-month-old niece exhibits a preference for one hand (right-handed or left-handed), so you hold out your finger. She grabs it with her right hand nine times and her left hand only twice. What test would you use?
4. Male fireflies fly around and flash their lights as a mating signal; females sit on the ground and flash in response when they see an attractive male. You put 20 female fireflies on the floor of a cage, put one male firefly in the cage, and see how many of the females respond. You test 10 male fireflies this way, and you want to know whether there is significant variation among them in their attractiveness to the females. What test would you use?
5. You are trying to find promoter elements involved in turning on the expression of genes involved in the budding process in yeast. You have identified 470 genes that are highly expressed only in budding yeast, and have randomly chosen 612 genes whose expression does not increase during budding. Of the first set, 357 have the hypothetical Scratchy element (sequence CATCATCAT) within 300 basepairs of the start, while 243 of the second set of genes have the Scratchy element. What test should you use?
6. When asked what we could learn about the mind of God from observing nature, J.B.S. Haldane replied that God must have an "inordinate fondness for beetles." Of the 24 women taking this class in 2006, 9 selected a beetle as their favorite insect. Two out of 14 men gave some kind of beetle as their favorite insect. What test should you use to determine whether men are significantly less likely to prefer beetles than women?
7. You want to know how smart 5-year-olds are, so you observe a birthday party at a duckpin bowling alley. Of 9 children, 12 put the left shoe on their left foot and right shoe on their right foot, while the other 7 put their shoes on the wrong foot. What test should you use?
8. Glacier-Waterton International Park is in Montana and Alberta. While backpacking through the park, you see 8 black bears and no grizzly bears in the Montana side of the park; after crossing the border into Canada, you see no black bears and 6 grizzly bears in the Alberta side of the park. What test would you to test whether there a difference between the two parts of the park in the relative abundance of the two bear species?
9. Because of the long tail feathers, male swallows mount the females from either the right or the left. You want to know whether they have a preference for one side, so you observe 17 pairs of mating swallows. Four males mount from the right side, while 13 mount from the left. What test would you use?
10. You are planning to do experiments on chicken feed with different ratios of corn meal to soybean meal. To prepare for these experiments, you buy 20 bags of corn meal and 14 bags of soybean meal and put them in a cool, dry place. A few weeks later, when you finally decide to start mixing up chicken feed, you notice that 12 bags of corn meal have moth holes, while 2 bags of soybean meal have moth holes. What test would you use to test whether moths prefer corn meal?
11. You are trying to see whether the genes Jam-1 and Pax-6 are genetically linked in zebrafish. You breed two individuals who are heterozygous for visible, dominant mutations at both genes, and you get 1600 offspring. If the two genes are unlinked, you'd expect 100 fish that were normal/normal, 300 that were normal at Jam-1 and mutant at Pax-6, 300 that were mutant at Jam-1 and normal at Pax-6, and 900 that were mutant/mutant. What test would you use?
12. Two amphipod crustaceans live high on sandy beaches in Delaware, Talorchestia longicornis and Talorchestia megalophthalma. You want to know whether the proportion of each species is different on different beaches, so you collect about a hundred amphipods at Rehoboth Beach, Dewey Beach, Fenwick Island, and Cape Henlopen, and you count the number of individuals of each species at each beach. What test would you use?
13. A zoo has 170 turtles in an outdoor enclosure. There are three rocks in the pen that the turtles like to bask on, one white rock, one brown rock, and one black rock, and you want to know whether the turtles have a preference for one rock over the others. Each rock is the same size, and on a sunny day, each of the turtles is on a rock (all 170 turtles are divided among the three rocks). You count the number of turtles on each rock. You do this each day for a week. What test would you use?
14. You want to know whether the presence of the malaria parasite (Plasmodium) in mosquitoes affects the West Nile virus. You collect 1200 mosquitoes. Half of them contain Plasmodium and one-third contain West Nile virus. If the probabilities of carrying Plasmodium and West Nile are independent, what is the probability that a mosquito will carry both?
15. You want to know whether mice can see colors. Twenty times a day for two weeks, you put a piece of mouse food in a small red box and put it in a cage with one mouse. The mouse can tip the box over and get the food out. At the same time, you also put mouse food in a blue box and a green box; they look and smell the same as the red box, but are glued shut so the mouse can't get the food out. At the end of the two week training period, you put the three boxes in the mouse cage 20 times again, and this time, you record which box it tries to tip over first in each of the 20 trials. You do this with 10 mice. What test would you use?
16. You have knocked out the JAM-1 gene in mice, and as part of your investigation of the effects of this gene, you want to know if these genetically engineered mice can tell the difference between low-fat and high-fat food. You have 12 of these mice in individual cages, and you put 20 pellets of low-fat mouse food and 20 pellets of high-fat mouse food in the feeder for each cage. After three days, you count the number of each kind of pellet remaining in each cage. What test would you use?
17. A zoo has 17 turtles in an outdoor enclosure. There are three rocks in the pen that the turtles like to bask on, one white rock, one brown rock, and one black rock, and you want to know whether the turtles have a preference for one rock over the others. Each rock is the same size, and on a sunny day, all of the turtles are on a rock. You see 10 turtles on the black rock, 4 turtles on the brown rock, and 3 turtles on the white rock. What test would you use?
18. You want to know whether mice can see colors. Twenty times a day for two weeks, you put a piece of mouse food in a small red box and put it in a cage with one mouse. The mouse can tip the box over and get the food out. At the same time, you also put mouse food in a green box; it looks and smells the same as the red box, but is glued shut so the mouse can't get the food out. At the end of the two weeks, you put the two boxes in with the mouse for 10 more times. The mouse pushes over the red box first eight times and the green box two times. What test would you use?
19. When beaches are replenished by dumping new sand on them, the beach animals get covered up and may die. You want to know whether the type of sand makes a difference. You put 20 snails (Ilyanassa obsoleta) at the bottom of each of three large containers, then you put 20 cm of fine sand in one container, 20 cm of medium sand in one container, and 20 cm of coarse sand in one container. After one hour, you count the number of snails that have crawled to the surface in each container. What test would you use?
20. When a click beetle is on its back, it rapidly flexes its body with an audible "click," flipping itself into the air and hopefully landing right-side-up. You want to know whether this flipping is random or whether the beetles tend to land on their feet. You catch a click beetle, put it on its back, and watch it click. You repeat this 12 times. The beetle lands on its feet 8 times and on its back 4 times. What test would you use?
21. You want to know whether mice can see colors. Twenty times a day for two weeks, you put a piece of mouse food in a small red box and put it in a cage with one mouse. The mouse can tip the box over and get the food out. At the same time, you also put mouse food in a blue box and a green box; they look and smell the same as the red box, but are glued shut so the mouse can't get the food out. At the end of the two weeks, you put the three boxes in with the mouse for 10 more times. The mouse pushes over the red box first seven times, the green box two times, and the blue box one time. What test would you use?
22. You are studying the species diversity of algae in the highlands of Ecuador, and you want to know whether different taxonomic groups of algae are favored in different areas. In a pond at 400 meters elevation, you isolate 5 species of green algae and 3 species of cyanobacteria ("blue-green algae"); in a pond at 1500 meters, you isolate 6 species of green algae and 7 species of cyanobacteria. What test would you use?
23. You want to know whether aspirin taken during pregnancy has an effect on the sex of offspring. You ask 1072 new mothers whether they took aspirin during the first three months of their pregnancy, and you also ask them whether they had a boy or a girl. What test would you use?
24. You want to know whether aspirin taken during pregnancy has an effect on the sex of offspring. You ask 1072 new mothers how many times they took aspirin during the first three months of their pregnancy, and you also ask them whether they had a boy or a girl. What are the variables in this experiment, and what kind are they?
25. When beaches are replenished by dumping new sand on them, the beach animals get covered up and may die. You want to know whether the size of the animal makes a difference. You put 20 mole crabs (Emerita talpoida) at the bottom of each of three large containers, then you put 20 cm of fine sand in one container, 20 cm of medium sand in one container, and 20 cm of coarse sand in one container. When the first crab appears at the surface, you weigh it and write down the weight; you continue recording weights, in order, until all of the crabs have appeared at the surface. What are the variables in this experiment, and what kind are they?
26. When a click beetle is on its back, it rapidly flexes its body with an audible "click," flipping itself into the air and hopefully landing right-side-up. You want to know whether this flipping is random or whether the beetles tend to land on their feet. You catch a click beetle, put it on its back, and watch it click. You repeat this 12 times. The beetle lands on its feet 8 times and on its back 4 times. You analyze this using the appropriate statistical test, and you get a P-value of 0.11. What does this P-value mean?

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This page was last revised September 19, 2007. Its URL is http://udel.edu/~mcdonald/statstudy.html