1. An experiment was conducted to test the effect of humidity on plant transpiration by weighing plants before and after exposure to different humidity levels. Which factor introduces a confounding variable?
2. Researchers tested the effect of sunlight on plant growth by placing 20 plants under direct sunlight and 20 plants in shaded areas for 4 weeks. Height was measured weekly. Which of the following is the best criticism of this experiment?
3. In an experiment, students tested the effect of fertilizer on plant growth by applying varying amounts to different plants while keeping sunlight constant. What additional trial would most improve the experiment?
4. Researchers tested the effect of exercise on weight loss by having participants perform daily workouts for 6 weeks. What additional trial would most improve the experiment?
5. A student tested whether salt lowers the freezing point of water. Three beakers each received 200 mL of water. The independent variable was salt mass added: 0 g, 5 g, or 10 g. The dependent variable was the temperature when the first ice crystals appeared, measured with a thermometer. The student placed all beakers in the same freezer at the same time. The beakers were uncovered, and the freezer door was opened briefly every 5 minutes to check for ice. Each condition was tested once. The student concluded that increasing salt always decreases freezing temperature.
Which change would make the experiment more reliable?
6. A student studies the effect of different colored lights on plant photosynthesis. The independent variable is light color, and the dependent variable is the rate of photosynthesis. Which factor introduces a confounding variable?
7. An experiment investigates the effect of different liquids on metal corrosion. The independent variable is the type of liquid, and the dependent variable is the rate of corrosion. Which of the following is the best criticism of this experiment?
8. A student examined whether paint color affects drying time. Four identical wooden boards were painted with the same brand of paint in different colors: white, red, blue, and black. The independent variable was color. The dependent variable was time to “dry,” defined as when the surface no longer felt sticky to the student’s finger. Boards were placed outdoors in sunlight. Results: white 55 min, red 60 min, blue 58 min, black 40 min. The student concluded black paint dries fastest because it absorbs more heat.
Which change would most improve measurement validity?
9. A class tested whether a new phone case reduces phone temperature during gaming. Two identical phones ran the same game for 20 minutes. Phone 1 used the new case; Phone 2 had no case. The dependent variable was maximum battery temperature recorded by a built-in sensor app. Both phones started at 100% charge. Results: Phone 1 max 39.2°C; Phone 2 max 41.0°C. The class concluded the case reduces temperature. However, Phone 1 was connected to Wi-Fi and Phone 2 used cellular data because Wi-Fi was weak near Phone 2.
Which variable was NOT controlled in this study?
10. A student tested the hypothesis that caffeine increases typing speed. Twenty volunteers were asked to type a 3-minute passage on the same laptop model. The independent variable was drink type: caffeinated tea (200 mg caffeine) or decaffeinated tea (0 mg). The dependent variable was words typed correctly per minute (WCPM). The student told all participants to avoid caffeine for 12 hours beforehand, but did not verify compliance. Participants chose their preferred drink type, then waited 10 minutes and typed. The room, passage, and keyboard settings were kept constant. Results: caffeinated group (n=14) mean 52 WCPM; decaf group (n=6) mean 49 WCPM. The student concluded caffeine increases typing speed.
Which of the following is the best criticism of this experiment?
11. A student investigated whether dehydration affects heart rate after exercise. Ten participants ran for 5 minutes, then heart rate was measured immediately. The independent variable was hydration condition: participants either drank 500 mL water 30 minutes before running or drank nothing. The dependent variable was heart rate. Participants chose which condition they preferred. Results: water group mean 142 bpm; no-water group mean 151 bpm. The student concluded dehydration increases heart rate.
Which change would make the experiment more valid?
12. A study was conducted to examine the effect of exercise on mood improvement. Participants engaged in either aerobic or anaerobic exercise for 30 minutes daily. Mood was self-reported using a survey. Which factor introduces a confounding variable?
13. A study examined the effect of different teaching methods on student performance. Students were taught using lectures, discussions, and projects. Which of the following is the best criticism of this experiment?
14. A scientist investigates the effect of salt concentration on the boiling point of water. The independent variable is the salt concentration, and the dependent variable is the boiling point. Which of the following is the best criticism of this experiment?
15. A student tested whether colored light affects plant growth. Identical seedlings were placed under red, blue, or white LED lamps for 14 days. The independent variable was light color. The dependent variable was final plant mass. The student set all lamps to the same electrical power (10 W) and placed plants the same distance from each lamp. Results: red 3.1 g, blue 2.4 g, white 2.9 g. The student concluded red light produces the most growth. A teacher noted that equal electrical power does not guarantee equal light intensity (photon flux) across colors.
Which of the following is the best criticism of this experiment?
16. Researchers tested the effect of different study techniques on memory retention by using flashcards, summaries, and practice tests. What additional trial would most improve the experiment?
17. A study aimed to determine the effect of diet on energy levels by comparing high protein and high carbohydrate diets. Which factor introduces a confounding variable?
18. A student tested whether a filter pitcher removes lead from water. They prepared one sample of tap water and measured lead concentration with a test strip (reported in ppm). Then they filtered the same water once and measured lead again. Lead was 0.08 ppm before and 0.04 ppm after. The student concluded the filter removed 50% of lead. The test strip packaging states readings vary by $\pm 0.03$ ppm and should be averaged over at least 3 strips per sample.
Which change would make the experiment more reliable?
19. A student examines the effect of sugar type on cookie texture. The independent variable is sugar type, and the dependent variable is cookie texture score. Which factor introduces a confounding variable?
20. To better test the hypothesis that hydration affects cognitive performance, the student should:
21. A scientist examined the effect of different wavelengths of light on photosynthesis by exposing plants to red, blue, and green light. What additional trial would most improve the experiment?
22. Which change would make a study about the effect of diet on skin health more reliable?
23. A classroom experiment investigated the effect of water temperature on dissolving sugar. Students used hot, warm, and cold water. What change would make the experiment more reliable?
24. To better test the hypothesis that music affects plant growth, the student should:
25. A student tested whether a new alarm sound helps people wake faster. Ten participants slept in a dorm. On Night 1, the standard alarm sounded; on Night 2, the new alarm sounded. The dependent variable was time from alarm start to standing, measured by the participant with a phone stopwatch. Mean time decreased from 90 s to 55 s. The student concluded the new alarm is better. The student did not randomize alarm order, and participants knew which alarm was being tested.
Which change would most improve the experiment?
26. A student examined whether a new running shoe reduces 1-mile time. Eight runners ran 1 mile in their old shoes on Monday and in the new shoes on Friday. The dependent variable was mile time. Mean time decreased from 8:10 to 7:55. The student concluded the new shoe improves performance. The student did not record weather conditions, and Monday was cool and cloudy while Friday was hot and windy.
Which factor introduces a confounding variable?
27. A lab group investigated whether fertilizer concentration affects basil growth. They planted 6 basil seedlings per treatment in identical pots with the same soil mass and placed all pots on one greenhouse bench. The independent variable was fertilizer concentration in water: 0%, 1%, or 2% (by volume). The dependent variable was plant height after 21 days. Each pot received 100 mL of solution every other day. Results (mean height): 0% = 12 cm, 1% = 18 cm, 2% = 17 cm. The group concluded 1% fertilizer is optimal. However, pots were arranged by treatment in three clusters on the bench: all 0% on the left, 1% in the middle, 2% on the right.
Which factor introduces a confounding variable?
28. A study examined the effect of different music genres on heart rate. Participants listened to classical, rock, and jazz music. Which of the following is the best criticism of this experiment?
29. An experiment was conducted to investigate the effect of temperature on yeast fermentation by measuring CO2 production at different temperatures. Which factor introduces a confounding variable?
30. A student tests the hypothesis that caffeine increases heart rate. The experiment measures heart rate before and after consuming a caffeinated drink. Which factor introduces a confounding variable?
31. A physics student tested whether ramp angle affects the distance a toy car travels after leaving the ramp. The independent variable was ramp angle: 15°, 30°, or 45°. The dependent variable was horizontal distance traveled on the floor after leaving the ramp. The student used the same ramp and car, releasing the car from the top each time. However, the floor surface changed: 15° trials were on carpet, 30° on tile, and 45° on wood, because of space constraints. The student concluded steeper ramps increase distance.
Which of the following is the best criticism of this experiment?
32. Which change would make a study about the effect of light intensity on photosynthesis more reliable?
33. To better test the hypothesis that sleep duration affects cognitive performance, the student should:
34. An experiment is conducted to test the effect of air pressure on sound speed. The independent variable is air pressure, and the dependent variable is the speed of sound. Which change would make the experiment more reliable?
35. A study investigates the effect of light intensity on the rate of photosynthesis in aquatic plants. The independent variable is light intensity. The dependent variable is oxygen production. Which change would make the experiment more reliable?
36. Which change would make a study about the effect of noise on concentration more reliable?
37. An experiment was conducted to test the effect of diet on cholesterol levels by assigning participants different diets and measuring their cholesterol after 4 weeks. Which factor introduces a confounding variable?
38. An experiment tests the effect of water temperature on dissolving sugar. The independent variable is water temperature, and the dependent variable is time taken for sugar to dissolve. What additional trial would most improve the experiment?
39. An experiment tests the effect of humidity on mold growth. The independent variable is humidity level, and the dependent variable is mold coverage area. Which of the following is the best criticism of this experiment?
40. To better test the hypothesis that water temperature affects the rate of chemical reactions, the student should:
41. An experiment tests the effect of exercise type on heart rate. The independent variable is exercise type, and the dependent variable is heart rate increase. What additional trial would most improve the experiment?
42. Which change would make a study about the effect of exercise on mental health more reliable?
43. Researchers examine the impact of different diets on mice weight gain. The independent variable is diet type, and the dependent variable is weight gain. To better test the hypothesis, the researchers should:
44. An experiment is conducted to study the effect of soil pH on plant growth. The independent variable is soil pH, and the dependent variable is plant biomass. What additional trial would most improve the experiment?
45. Researchers tested the effect of study environment on test performance by having students study in quiet, noisy, and music-filled environments. What additional trial would most improve the experiment?
46. Researchers test the effect of sugar concentration on yeast fermentation. The independent variable is sugar concentration, while the dependent variable is carbon dioxide production. To better test the hypothesis, the researchers should:
47. Two students tested whether a magnet affects the rate of a chemical reaction that produces gas bubbles (effervescence) from a tablet in water. They placed one tablet in a beaker 10 cm from a strong magnet and another tablet in a beaker 1 m from the magnet. The independent variable was distance from the magnet. The dependent variable was time until bubbling stopped, measured with a stopwatch. Both beakers used 200 mL of tap water. Results: 10 cm = 95 s; 1 m = 120 s. They concluded magnets speed the reaction. However, the beaker near the magnet was stirred continuously to keep it from tipping, while the other was not stirred.
Which factor introduces a confounding variable?
48. A chemist tested whether stirring affects how fast sugar dissolves. Two cups each received 200 mL water and 10 g sugar. Cup 1 was stirred continuously; Cup 2 was not stirred. The dependent variable was time until the sugar was “fully dissolved,” determined by looking for undissolved crystals. Cup 1 dissolved in 40 s; Cup 2 in 140 s. The chemist concluded stirring speeds dissolution. However, Cup 1 used hot water (about 60°C) while Cup 2 used room-temperature water (about 22°C).
Which variable was NOT controlled in this study?
49. A researcher examined whether listening to music improves memory recall. Participants studied a list of 30 words for 2 minutes, then recalled as many as possible. The independent variable was audio condition during studying: silence or instrumental music at 60 dB. The dependent variable was number of words recalled. The researcher tested 8 participants in silence in the morning and 8 participants with music in the afternoon. The same room and word list were used. Results: silence mean 18 words; music mean 22 words. The researcher concluded music improves recall.
Which of the following is the best criticism of this experiment?
50. A biology club tested whether a sugar solution increases yeast fermentation rate. They prepared two flasks: Flask S contained 100 mL of 10% sugar solution and yeast; Flask W contained 100 mL of water and yeast. The dependent variable was CO$_2$ produced after 15 minutes, measured by the volume of gas in a balloon attached to each flask. Results: Flask S balloon circumference 28 cm; Flask W circumference 8 cm. The club concluded sugar increases fermentation. However, Flask S was placed on a warm lab bench near a heater, while Flask W was on a cooler table.
Which variable was NOT controlled in this study?
51. A student tested whether a sunscreen reduces UV exposure. Two identical UV-sensitive beads were placed outside for 10 minutes at noon. Bead A was coated with sunscreen; Bead B was uncoated. The dependent variable was bead color intensity after exposure, rated by the student on a 1–5 scale (5 = darkest). Results: Bead A rated 2; Bead B rated 4. The student concluded sunscreen reduces UV exposure. The student did not measure the thickness of sunscreen applied and noted it was “a thin layer.”
To better test the hypothesis, the student should:
52. An environmental science class tested whether road salt increases water conductivity. They collected one snowmelt sample from a roadside ditch and one sample from a park pond. The independent variable was “salt exposure” (ditch assumed high, pond assumed low). The dependent variable was conductivity measured with a probe. Results: ditch 2100 $\mu$S/cm; pond 450 $\mu$S/cm. The class concluded road salt increases conductivity. However, the two samples came from different water bodies with unknown differences besides salt.
Which of the following is the best criticism of this experiment?
53. An experiment is designed to test the effect of sound frequency on plant growth. The independent variable is sound frequency, and the dependent variable is plant height. Which change would make the experiment more reliable?
54. In a study on the impact of screen time on sleep quality, participants reported their daily screen usage and sleep patterns. Which of the following is the best criticism of this experiment?
55. To better test the hypothesis that stress affects memory recall, the student should:
56. Researchers analyze the impact of soil moisture on seed germination. The independent variable is soil moisture level, and the dependent variable is germination rate. To better test the hypothesis, the researchers should:
57. A student tested whether a disinfectant spray reduces bacterial growth on countertops. Two 10 cm$\times$10 cm countertop squares were swabbed after 24 hours in a classroom. Square A was sprayed with disinfectant once; Square B was not sprayed. Swabs were streaked onto agar plates, incubated 48 hours, and colony counts were recorded: A = 12 colonies, B = 60 colonies. The student concluded the spray reduces bacteria. The student sampled only one treated square and one untreated square.
What additional trial would most improve the experiment?
58. A team tested whether a new study app improves quiz scores. Ten students took Quiz 1 without the app, then used the app for one week and took Quiz 2. The independent variable was app use (before vs. after). The dependent variable was quiz score. Quiz 1 and Quiz 2 covered different chapters and were written by different teachers. Mean score increased from 72% to 81%. The team concluded the app caused the improvement.
Which of the following is the best criticism of this experiment?