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Experimental Design - ACT Science

  1. In a study on the impact of diet on cholesterol levels, participants followed different diets for 6 months, and cholesterol levels were measured. What is a possible improvement?
    Answer: Include a diverse participant demographic. The study tests how different diets affect cholesterol levels by having participants follow various diets for 6 months and measuring cholesterol changes. The current design may be limited by participant demographics, which could affect the generalizability of results across different populations. Including a diverse participant demographic would improve the study by ensuring results apply broadly across age groups, ethnicities, and health conditions. Diverse demographics would strengthen the external validity of findings and make recommendations more applicable to the general population rather than a narrow participant group.
  2. In a study measuring the effect of study duration on test performance, students study various lengths of time and take the same test. What is the independent variable?
    Answer: Study duration The hypothesis is that study duration affects test performance in students. The experimental design varies the amount of time students spend studying before taking the same test, making study duration the independent variable - the factor being systematically manipulated by researchers. Test performance would be the dependent variable (what's measured), while factors like test difficulty and student age would ideally be controlled. Study duration serves as the time variable being tested for its impact on academic outcomes.
  3. A scientist tested whether adding yeast increases CO2_2 production in bread dough. Materials: flour, water, salt, sugar, yeast, 6 sealed bottles with balloons, balance. Procedure: (1) Prepare two dough mixtures: Mix Y contains yeast; Mix N is identical but without yeast. (2) Place 50.0 g of each mixture into 3 bottles each. (3) Stretch a balloon over each bottle opening to capture gas. (4) After 60 min at 30°C, measure balloon circumference as an estimate of gas produced. Results: Mix Y balloons were larger than Mix N. The primary purpose of this experiment was to:
    Answer: Determine whether yeast produces CO2_2 gas during dough fermentation compared with an otherwise identical mixture. The primary purpose of this experiment was to determine whether yeast produces CO₂ gas during dough fermentation compared with an otherwise identical mixture. The experiment uses yeast presence/absence as the variable while measuring gas production through balloon inflation, directly testing yeast's role in CO₂ generation during fermentation. Choice D correctly identifies this hypothesis: comparing gas production between yeast and no-yeast conditions. Choice B incorrectly focuses on salt's role, when salt is present in both mixtures as a controlled variable.
  4. A researcher tested whether activated charcoal removes dye from water. Materials: 6 clear cups, blue dye solution, activated charcoal powder, coffee filters, and a balance. Procedure: (1) Add 100 mL of the same dye solution to each cup. (2) Add charcoal to reach 0.0 g, 0.5 g, or 1.0 g (2 cups per amount). (3) Stir each cup for exactly 30 s. (4) Filter each mixture through a coffee filter into a clean cup. (5) Measure the filtered liquid's color intensity using a phone app that reports a "blue value" from 0 (no blue) to 255 (very blue). Lower blue values indicate more dye removal. Step 4 (filtering) was included in order to:
    Answer: Remove charcoal particles so the color measurement reflects dissolved dye, not suspended black powder. The purpose of filtering is to remove the charcoal particles from the solution before measuring color intensity. The experiment tests whether charcoal removes dye by adsorption, but the presence of black charcoal powder in the liquid would interfere with accurate color measurement. Filtering separates the solid charcoal (which may have adsorbed dye) from the liquid phase, allowing the app to measure only the dissolved dye remaining in solution. Answer A correctly identifies this purpose - removing charcoal particles so color measurement reflects dissolved dye concentration. The filter doesn't concentrate dye or chemically convert it.
  5. A medical intern tested whether hand sanitizer reduces bacterial growth on hands. Materials: agar plates, sterile swabs, hand sanitizer (70% ethanol), soap and water, incubator. Procedure: (1) Swab one hand before cleaning and streak onto Plate B (before). (2) Wash both hands with soap and water for 20 s. (3) Apply sanitizer to one hand only; let dry 30 s. (4) Swab the sanitized hand and streak onto Plate S; swab the non-sanitized hand and streak onto Plate W. (5) Incubate 48 h and count colonies. Result: Plate S had fewer colonies than Plate W. A flaw in the experimental design is that:
    Answer: Only one person was tested, so individual variability could limit conclusions about sanitizer effectiveness. The experimental design flaw is that only one person was tested, so individual variability could limit conclusions about sanitizer effectiveness. Personal factors like skin type, baseline bacteria levels, hand washing technique, or immune responses could affect results in ways that don't generalize to other people, making the sample size inadequate for drawing broader conclusions. Choice A correctly identifies this sample size limitation. Choice C incorrectly suggests soap eliminates all bacteria, when the purpose is comparing sanitized versus non-sanitized hands after identical initial cleaning.
  6. A physicist tested the effect of wire length on electrical resistance. Wires of 1m, 2m, and 3m were used, and resistance was measured. The primary purpose of this experiment was to:
    Answer: Determine how wire length affects resistance. The experiment tests how wire length affects electrical resistance by using wires of different lengths (1m, 2m, 3m) and measuring resistance values. The experimental design manipulates wire length while keeping other factors constant to isolate length's effect on resistance. The primary purpose is to determine how wire length affects resistance, as this directly addresses the relationship between these two variables. The experiment specifically varies length to test this electrical property relationship rather than comparing materials or measuring other electrical characteristics.
  7. An experiment was conducted to determine if a special coating on cookware affects heat distribution. Cookware with and without coating was heated, and surface temperatures were measured. What is a potential design flaw?
    Answer: Surface temperature should be measured at multiple points. The experiment tests whether a special coating affects heat distribution in cookware by comparing coated versus uncoated surfaces. For accurate heat distribution assessment, temperature should be measured at multiple locations across each cookware surface rather than at single points. Surface temperature should be measured at multiple points because heat distribution refers to how evenly temperature spreads across the surface, requiring multiple measurements to detect hot spots or uneven heating patterns. Single-point measurements could miss important distribution differences between coated and uncoated cookware.
  8. In a study on the effect of screen time on sleep quality, participants reported screen usage and sleep quality over a month. A flaw in the experimental design is that:
    Answer: Participants self-reported their sleep quality. The study examines the relationship between screen time and sleep quality by having participants report both variables over a month-long period. The experimental design relies entirely on participant self-reporting for sleep quality assessment, which introduces subjective bias and potential inaccuracy. A flaw in the experimental design is that participants self-reported their sleep quality, as subjective sleep assessments can be influenced by personal perceptions, expectations, and recall bias. Objective sleep measurements using devices or controlled sleep studies would provide more reliable data.
  9. In an experiment, researchers wanted to determine the effect of sunlight on plant growth. They placed 20 plants in a greenhouse, with 10 plants receiving 6 hours of sunlight per day and the other 10 receiving no sunlight. At the end of four weeks, they measured the height of each plant. What is the independent variable in this experiment?
    Answer: Amount of sunlight received The experiment tests whether sunlight exposure affects plant growth by comparing plants receiving different amounts of sunlight. The researchers manipulate the amount of sunlight (6 hours vs. no sunlight) while keeping other factors constant, allowing them to isolate sunlight's effect on growth. The independent variable is the amount of sunlight received because this is the factor being deliberately varied by the researchers. Duration and plant type are held constant, while height is the measured outcome (dependent variable).
  10. A biologist observes bird feeding patterns at different times of day. What is the dependent variable in this study?
    Answer: Feeding patterns The hypothesis is that time of day affects bird feeding behavior. In this observational study, the researcher manipulates the time of observation (independent variable) and measures the resulting feeding patterns (dependent variable). Feeding patterns represent what is being observed and recorded as the outcome, making it the dependent variable that responds to changes in timing. The dependent variable is always what researchers measure or observe as a response to changes in the independent variable.
  11. A teacher tested whether rinsing rice reduces starch in the cooking water. Materials: 6 identical pots, 300 g uncooked white rice per pot, water, sieve, timer, colorimeter, iodine solution. Procedure: (1) For 3 pots, rinse rice under running water for 60 s, then drain. For 3 pots, do not rinse. (2) Add 600 mL water to each pot and boil for 12 min with lids on. (3) Collect 10 mL cooking water from each pot and add 2 mL iodine solution. (4) Measure absorbance at 620 nm; higher absorbance indicates more starch. Results: rinsed mean absorbance = 0.35; not rinsed = 0.62. The primary purpose of Step 3 was to:
    Answer: Create a colored complex with starch so starch amount can be estimated by absorbance. The primary purpose of Step 3 was to create a colored complex with starch so starch amount can be estimated by absorbance. Iodine forms a characteristic blue-black complex with starch, and the intensity of this color (measured as absorbance) directly correlates with starch concentration in the cooking water. Choice B correctly identifies this analytical purpose: iodine creates a measurable colored indicator of starch presence. Choice A incorrectly suggests converting starch to sugar, when iodine actually forms a detection complex.
  12. Scientists are testing whether a new fertilizer increases crop yield compared to no fertilizer. A flaw in this experimental design is that:
    Answer: There is no randomization of plots. The hypothesis is that the new fertilizer increases crop yield compared to no fertilizer treatment. A critical flaw in this experimental design is the lack of randomization of plots, which could lead to bias if certain plots have inherently better or worse growing conditions. Without randomizing which plots receive fertilizer versus control treatment, any differences in yield might be due to plot location, soil quality, or other environmental factors rather than the fertilizer itself. Randomization is essential to ensure that treatment and control groups are comparable.
  13. Researchers tested the effect of light intensity on plant growth using three groups of plants exposed to low, medium, and high light. What is the independent variable in this experiment?
    Answer: Light intensity The hypothesis being tested is that light intensity affects plant growth rates. The experimental design manipulates light intensity (low, medium, high) as the factor being changed by the researchers, making it the independent variable - the variable that is deliberately controlled and varied to test its effect. Plant growth would be the dependent variable (what is measured), while factors like number of plants and watering frequency would be controlled variables. The independent variable is always the factor that researchers intentionally manipulate to observe its effects.
  14. Researchers asked whether caffeine increases the heart rate of freshwater fish (zebrafish). They prepared 4 aquariums (10 fish each) with identical volume (20 L), temperature (25°C), light cycle, and feeding schedule. Step 1: Fish were acclimated for 24 hr in plain water. Step 2: Baseline heart rate was measured for each fish by recording a 15-s microscope video and counting beats, then converting to beats/min. Step 3: Each aquarium received caffeine dissolved in water to reach 0, 10, 20, or 40 mg/L; fish remained in the solution for 30 min. Step 4: Heart rate was measured again using the same 15-s video method. The independent variable was caffeine concentration; the dependent variable was change in heart rate from baseline. Each aquarium's mean change was calculated. The researchers expected higher caffeine concentrations to produce larger increases in heart rate. Which hypothesis is being tested?
    Answer: Caffeine increases zebrafish heart rate, and the increase becomes larger at higher caffeine concentrations. The hypothesis being tested is that caffeine increases zebrafish heart rate, with larger increases at higher concentrations. The experiment systematically varies caffeine concentration (0, 10, 20, or 40 mg/L) while keeping all other factors constant - aquarium size, temperature, light cycle, and feeding schedule. By measuring heart rate before and after caffeine exposure, the researchers can isolate caffeine's effect on heart rate. Answer A correctly captures this hypothesis by stating that caffeine increases heart rate and the effect is dose-dependent. The other options propose alternative hypotheses about aquarium size, metabolism, or temperature that are not being tested in this controlled experiment.
  15. A student tested whether light color affects photosynthesis rate in an aquatic plant (Elodea). The student hypothesized that red light produces a higher photosynthesis rate than green light at the same intensity. Procedure: (1) Place a 10 cm Elodea sprig in a test tube containing 40 mL of 0.2% sodium bicarbonate solution. (2) Position an LED lamp 20 cm from the tube; use a light meter to adjust lamp settings so intensity is 500 lux at the tube for each color. (3) Cover the tube with a clear plastic wrap to minimize gas exchange with air. (4) Illuminate for 10 minutes using either red, green, or white LEDs. (5) Count oxygen bubbles released from the cut stem for the final 5 minutes; record bubbles/min. (6) Repeat 5 trials per light color, using a new sprig each trial. Results were compared among colors. Step 3 was included in order to:
    Answer: Reduce exchange of gases with air so measured bubbles reflect plant oxygen production. The experiment measures photosynthesis rate by counting oxygen bubbles produced by the plant under different light colors. Step 3 covers the tube with clear plastic wrap to minimize gas exchange with air, ensuring that the oxygen bubbles counted come from the plant's photosynthesis rather than from dissolved atmospheric gases. This step reduces exchange of gases with air so measured bubbles reflect plant oxygen production, making the bubble count a more accurate measure of photosynthesis rate. Without this step, oxygen from the air could dissolve into or escape from the solution, confounding the results.
  16. Students tested whether adding salt to ice water lowers the temperature enough to cool a soda faster. They used 12 identical 355 mL cans of room-temperature soda (22°C), 2 insulated beakers (2 L), crushed ice, water, table salt, a digital thermometer (±0.1°C), and a stopwatch. Procedure: (1) Fill each beaker with 1.0 L ice and 0.5 L water. (2) Add 200 g salt to Beaker S; add 200 g sugar to Beaker U to keep added mass similar. Stir each beaker 30 s. (3) Place 6 cans in each beaker, fully submerged, and start timing. (4) After 5, 10, and 15 min, remove one can from each beaker and immediately measure soda temperature by inserting a probe through a small puncture in the can. (5) Record temperatures and discard punctured cans. Results (mean soda temperature at 10 min): Beaker S = 6.8°C; Beaker U = 10.9°C. Which hypothesis is being tested?
    Answer: Salt lowers the ice-water mixture temperature, cooling submerged soda cans faster than a non-salt additive. The hypothesis being tested is that salt lowers the ice-water mixture temperature enough to cool soda faster than other additives. The experiment compares salt (which lowers freezing point and creates a colder mixture) against sugar as a control for added mass, while measuring soda temperature after identical cooling times. Choice B correctly identifies this hypothesis: salt's ability to lower mixture temperature results in faster cooling compared to a non-salt additive. Choice D incorrectly focuses on viscosity rather than the freezing point depression that actually causes the temperature difference.
  17. A physics class tested whether a parachute's canopy area affects fall time. Materials: 9 identical toy figures (same mass), plastic bags, string, scissors, meter stick, stopwatch, 3 m drop height. Procedure: (1) Build parachutes with canopy areas 100 cm2^2, 200 cm2^2, and 400 cm2^2 (3 parachutes each). (2) Attach each parachute to a toy with equal string length. (3) Drop each toy from 3 m indoors and time from release to floor contact. (4) Repeat each drop twice and average times. Results: larger canopy area produced longer fall times. What is the independent variable in this experiment?
    Answer: The canopy area of the parachute, changed across three designed size conditions. The independent variable is the canopy area of the parachute, changed across three designed size conditions. This is the factor deliberately manipulated by the researchers (100, 200, and 400 cm²) to test its effect on fall time while other variables like drop height, toy mass, and string length remain constant. Choice B correctly identifies canopy area as the independent variable. Choice D describes the dependent variable (fall time), while choice C describes a controlled variable.
  18. A student tested the effect of different soil types on plant growth. They planted seeds in clay, sandy, and loamy soils and measured plant height after 6 weeks. The primary purpose of this experiment was to:
    Answer: Find the best soil for plant growth. The experiment tests how different soil types affect plant growth by comparing growth outcomes across three soil conditions. By planting seeds in clay, sandy, and loamy soils and measuring plant height, the student can determine which soil produces the best growth results. The primary purpose is to find the best soil for plant growth, as this involves comparing multiple soil types to identify the most effective one. The other options focus on specific soil properties rather than overall growth performance.
  19. A lab group investigated whether a caffeine solution increases heart rate in Daphnia (water fleas). Hypothesis: higher caffeine concentration increases Daphnia heart rate. Procedure: (1) Prepare caffeine solutions at 0.0% (control), 0.1%, and 0.5% (mass/volume) in pond water. (2) Place one Daphnia in a depression slide with 1 mL of solution and allow 2 minutes to acclimate. (3) Under a microscope, count heartbeats for 15 seconds and multiply by 4 to estimate beats/min. (4) Repeat for 10 Daphnia per concentration. (5) The same student performed all counts to reduce observer variation. The group compared mean beats/min among concentrations. A flaw in the experimental design is that:
    Answer: Water temperature was not controlled or recorded during microscope observations. The experiment tests whether caffeine concentration affects Daphnia heart rate by comparing different caffeine solutions. A critical flaw is that water temperature was not controlled or recorded during microscope observations, yet temperature is known to significantly affect heart rate in ectothermic organisms like Daphnia. The microscope light could heat the slide, and different observation times could lead to different temperatures, introducing an uncontrolled variable that could mask or exaggerate caffeine effects. Using different Daphnia (B) is actually appropriate for this design, and counting for 15 seconds (A) is a reasonable time-saving measure that doesn't constitute a flaw.
  20. In an experiment to see if sugar affects yeast fermentation, yeast cultures were grown with 5%, 10%, and 15% sugar concentrations. The amount of carbon dioxide produced was measured. The dependent variable is:
    Answer: Amount of carbon dioxide produced The experiment tests whether sugar concentration affects yeast fermentation by varying sugar levels (5%, 10%, 15%) and measuring the fermentation output. Fermentation produces carbon dioxide as yeast breaks down sugar, so CO2 production directly indicates fermentation activity. The dependent variable is the amount of carbon dioxide produced because this is the measured outcome that responds to changes in sugar concentration. Sugar concentration is the independent variable being manipulated, while CO2 production is the response being measured to assess fermentation rates.

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