A student tests how light color affects oxygen release by Elodea. A 10 cm sprig sits in each of four beakers at 22 degrees Celsius, lit from 30 cm by a red, blue, green or white bulb. Bubbles are counted for 5 minutes. What is the independent variable?

Bread slices were sealed in bags and kept at 10, 20 or 30 degrees Celsius for six days, five bags per temperature. The moldy patch on each slice was traced on grid paper. Which quantity is the dependent variable, and how was it measured?

A researcher tests whether a pond bacterium slows duckweed growth. Twelve identical trays get pond water containing the bacterium, and twelve more get the same pond water after boiling has killed it. Which trays are the control group, and why?

A student claims a plant food increases leaf number in basil, and reports the data shown. Every plant stood on the same shelf and received 50 mL of water a day. What is the main weakness of this investigation?

A class measured how many seeds germinated in soil watered with 0%, 2% or 4% salt solution, using 10 seeds in a single tray at each concentration. Which change would most improve the investigation, and what problem would it fix?

Brine shrimp eggs were placed in dishes of water at four salt concentrations, 200 eggs per dish, all held at 25 degrees Celsius under the same lamp for 48 hours. Which factor did the investigators deliberately change?

Students test whether caffeine changes heart rate in Daphnia. Each animal is watched under a microscope in pond water, then in 0.1% caffeine solution, both at 20 degrees Celsius. What is the dependent variable, and how is it obtained?

A student tests whether mouthwash stops bacterial growth. Twenty agar plates were each spread with the same culture, given a paper disc as shown, and incubated at 30 degrees Celsius for two days. Which plates form the control group, and why?

A gardener asks whether a new compost helps tomatoes. He puts the compost pots in a sunny bed and the plain-soil pots in a shaded bed, waters both daily, and after eight weeks the compost plants are taller. What is the main weakness here?

Researchers ask whether crowding affects tadpole growth. They put 5, 10, 20 or 40 tadpoles in identical 4 L tanks, give every tadpole 0.1 g of food daily, and weigh each one after 30 days. Which variable is the independent one?

Students recorded pulse rate after three activity levels, with eight students in each group and one minute of counting each time. Their means are shown. Which measurement is the dependent variable, and how was it taken?

A team tests whether a fungus stunts wheat seedlings. Both groups get the same seed batch, 100 mL of water a day, a 20 degrees Celsius greenhouse and the same pot size; only one group meets the fungus. What are those matching conditions called?

A student put 20 water fleas in each of three jars of pond water for 24 hours and obtained the results shown. She concludes that something in the pond water is killing water fleas. Which flaw most undermines her conclusion?

A student found that ladybugs ate more aphids on a warm day than on a cool day, but she watched the plants for 10 minutes on the warm day and 30 minutes on the cool day. Which change would most improve the investigation, and what would it fix?

An investigation asked how pH affects the enzyme catalase. Equal potato disks were dropped into buffered hydrogen peroxide at four pH values, three tubes at each value, all at 25 degrees Celsius. What is the independent variable here?

Two groups of mealworms were kept on bran alone or on bran with added yeast, 25 larvae per group, at 27 degrees Celsius. Each larva was weighed at the start and again after 14 days. What is the dependent variable and how is it found?

A group tested a plant extract on a weed. All pots held the same soil and got 50 mL of water a day, and the table lists what each group of pots was sprayed with. Which group is the control, and why?

To boost algae growth, a student gave one tank extra light and extra nitrate, and left a second tank with normal light and no added nitrate. The first tank grew more algae, and he concluded that nitrate causes the growth. What is the main weakness?

A student tested whether music affects pea growth. Both rooms had the same window, temperature and watering. He measured the 10 music-room plants with a ruler and the 10 quiet-room plants against a piece of string. Which flaw undermines the comparison?

A doctor tests whether a new pill reduces coughing. Patients who ask for the pill are given it, and the patients who do not ask make up the comparison group. Which change would most improve the study, and what would it fix?