- A geology team tested whether adding water to sand increases its angle of repose (the steepest stable slope). Sand was poured into a pile under controlled conditions. Trials were conducted with dry sand and with sand containing 5% water by mass.
Based on the data, which conclusion is most strongly supported?
Answer: Adding 5% water increases the mean angle of repose compared with dry sand under the tested conditions.
Adding 5% water increases the mean angle of repose from 31° to 38° compared to dry sand under the tested conditions. This 7-degree increase indicates that moisture creates cohesive forces between sand grains, allowing steeper stable slopes before material begins to slide. The higher angle suggests that small amounts of water act as a binding agent, increasing inter-particle friction and cohesion through capillary forces. However, this conclusion applies specifically to the 5% water content tested and may not extend to higher moisture levels where different behaviors might occur.
- An astronomy team measured the brightness of a variable star over 6 nights using the same telescope. Brightness was recorded as apparent magnitude (lower magnitude = brighter). Cloud cover was noted each night.
The authors concluded: "The star dimmed on Nights 3 and 4 due to intrinsic stellar variability."
Which statement best evaluates the validity of the author's conclusion?
Answer: Not fully supported: increased cloud cover could make the star appear dimmer, providing an alternative explanation.
The conclusion is not fully supported because increased cloud cover during Nights 3-4 could make the star appear dimmer, providing an alternative explanation for the higher magnitude readings. The data show that nights with 60-70% cloud cover correspond to magnitudes of 10.8-10.9 (dimmer), while nights with 5-15% cloud cover show magnitudes of 10.1-10.2 (brighter). Since clouds can attenuate starlight and reduce apparent brightness, the observed dimming could reflect atmospheric interference rather than intrinsic stellar variability. To establish true stellar dimming, observations would need to control for or correct for atmospheric conditions.
- A materials scientist tested whether adding carbon fibers increases the tensile strength of a polymer. Samples were prepared with 0%, 5%, or 10% carbon fiber by mass. For each composition, five samples were pulled until failure and the mean tensile strength was recorded.
According to the data, which of the following can be concluded?
Answer: Higher carbon fiber content is associated with higher mean tensile strength across the tested compositions.
Higher carbon fiber content is consistently associated with increased mean tensile strength across the tested compositions (0%, 5%, and 10%). The data show a clear upward trend: 48 MPa at 0% fiber, 62 MPa at 5% fiber, and 70 MPa at 10% fiber. Each addition of carbon fiber corresponds to higher strength values, suggesting the fibers effectively reinforce the polymer matrix. However, this conclusion is limited to the tested range (0-10%) and cannot be extrapolated to higher fiber percentages or other compositions without additional testing.
- A study examined whether listening to instrumental music improves memory recall. Forty students were assigned to either Music or Silence during a 15-minute study period. Students then completed a 30-word recall test. Researchers also recorded whether students reported being "well-rested" (≥7 hours sleep) or "tired" (<7 hours).
The authors concluded: "Instrumental music significantly improves memory recall for students."
The conclusion that the authors' claim is:
Answer: Not supported: the data show sleep status has a much larger effect than music, and no significance is demonstrated.
The claim is not supported because sleep status has a much larger effect on recall than music, and no statistical significance is demonstrated. The data show well-rested students recalled 7-8 more words than tired students (24-25 vs 17-18), while music provided only a 1-word advantage in each sleep group. This 1-word difference is minimal and could easily result from random variation rather than a meaningful effect. The authors cannot conclude 'significant improvement' without statistical testing, and the much larger sleep effect suggests individual factors outweigh any potential music benefit.
- Ecologists investigated whether adding artificial nest boxes increases the number of breeding pairs of a bird species in urban parks. Four parks received 20 nest boxes each (Box parks) and four similar parks received none (No-box parks). Breeding pairs were counted in spring before installation (Year 0) and the next spring (Year 1). One Box park underwent tree removal between years.
Authors concluded: "Nest boxes caused the population increase observed in Year 1."
Which of the following is a flaw in the author's reasoning?
Answer: They assume causation, but both groups increased; factors like year-to-year conditions could explain the change.
The flaw is that the authors assume causation when both groups increased, and other factors like year-to-year environmental conditions could explain the population changes. The data show both No-box parks (12→13 pairs) and Box parks (11→14 pairs) increased from Year 0 to Year 1, indicating favorable breeding conditions affected all parks. While Box parks had a slightly larger increase (3 vs 1 pair), this difference could result from natural population fluctuations, habitat quality differences, or the tree removal mentioned in one Box park. The lack of a true control group receiving no intervention prevents isolating the nest box effect from temporal confounding variables.
- Engineers evaluated whether a new tire tread (Tread N) improves braking distance on wet pavement. Ten cars of the same model were tested on a closed track. Each car performed two braking trials from 60 km/h: one with standard tires and one with Tread N. Testing occurred on two different days; Day 2 had heavier rainfall.
The authors concluded: "Tread N reduces wet braking distance under rainy conditions."
Which statement best evaluates the validity of the author's conclusion?
Answer: Valid: Tread N has shorter mean braking distance than standard tires on both test days.
The conclusion is valid because Tread N consistently shows shorter mean braking distances than standard tires on both test days despite different weather conditions. On Day 1 (light rain), Tread N averaged 27.5m versus 29.0m for standard tires, and on Day 2 (heavy rain), it averaged 33.8m versus 34.5m for standard tires. While heavy rain increased stopping distances for both tire types, the consistent pattern across different conditions supports the claim that Tread N improves wet braking performance. The within-day comparisons control for environmental factors like rainfall intensity.
- A lab tested whether a disinfectant (Solution D) reduces bacterial growth on countertops. Identical countertop squares were inoculated with bacteria and then treated with either Solution D or water (Control). After 4 hours, technicians counted colony-forming units (CFU). The experiment was repeated at two room humidities.
Based on the data, which conclusion is most strongly supported?
Answer: Solution D reduces CFU relative to water at both humidities tested, though humidity still affects overall CFU.
Solution D reduces CFU relative to water control at both humidity levels tested, though humidity still affects overall bacterial survival. At 30% humidity, Solution D yields 150 CFU compared to 1,200 for Control (87.5% reduction), and at 70% humidity, it yields 260 CFU versus 2,100 for Control (87.6% reduction). The consistently large reductions demonstrate Solution D's effectiveness across humidity conditions. However, humidity clearly influences bacterial growth since both treatments show higher CFU at 70% than 30% humidity, indicating environmental factors interact with but don't eliminate the disinfectant's antimicrobial effect.
- A survey recorded the number of hours students studied per week and their exam scores. Figure 2 shows a scatter plot of study hours versus exam scores. Which conclusion is most strongly supported by the data?
Answer: More study hours correlate with higher exam scores.
More study hours correlate with higher exam scores according to the scatter plot data. The figure would show a positive relationship between hours studied per week and exam performance, with data points generally trending upward from left to right. This demonstrates a correlation where students who invest more time in studying tend to achieve better scores. Option C makes an absolute guarantee claim that goes beyond what correlation data can support.
- A neuroscience lab studied whether blue-light exposure before bedtime affects sleep onset latency (minutes to fall asleep). Twenty participants followed a fixed bedtime for 5 nights. On Nights 1–2, participants used a blue-light filter on their phones; on Nights 3–5, the filter was off. Participants also reported daily caffeine intake.
The authors concluded: "Turning off the blue-light filter causes people to take longer to fall asleep."
Which statement best evaluates the validity of the author's conclusion?
Answer: Not fully supported: higher caffeine intake during filter-OFF nights could explain longer latency, confounding the blue-light effect.
The conclusion is not fully supported because higher caffeine intake during filter-OFF nights (165 mg vs 110 mg) could explain the longer sleep latency, confounding the blue-light effect. The data show increased latency when the filter was off (26 min vs 18 min), but caffeine consumption also increased substantially during the same period. Since caffeine is a known stimulant that delays sleep onset, the observed effect could result from dietary changes rather than blue-light exposure. The study design fails to control for this confounding variable, making it impossible to isolate the impact of screen filter status on sleep.
- A psychology experiment tested whether a brief mindfulness session reduces test anxiety. Sixty students were randomly assigned to either 10 minutes of guided mindfulness or quiet sitting. Anxiety was rated (0–100) immediately before and immediately after the session.
Based on the data, which conclusion is most strongly supported?
Answer: Mindfulness is associated with a larger mean reduction in anxiety than quiet sitting under these study conditions.
Mindfulness is associated with a larger mean reduction in anxiety than quiet sitting under these study conditions. Both groups started with similar anxiety levels (68-69) and both decreased after the 10-minute sessions, but mindfulness produced a greater reduction. The mindfulness group decreased by 14 points (69→55) compared to 8 points for quiet sitting (68→60). While both interventions showed benefits relative to baseline, the mindfulness session resulted in a 6-point greater improvement than the control condition, suggesting it may be more effective for acute anxiety reduction in this population.
- A sports scientist tested whether caffeine improves sprint performance. Sixteen athletes completed a 200 m sprint after consuming either caffeine (3 mg/kg) or a placebo in a crossover design one week apart. However, all athletes received caffeine in Week 1 and placebo in Week 2. Weather differed: Week 1 had a tailwind; Week 2 had no wind.
Which statement best evaluates the validity of the author's conclusion that "Caffeine caused faster sprint times"?
Answer: Invalid: treatment is confounded with week and wind, so faster Week 1 times cannot be attributed to caffeine alone.
The conclusion is invalid because treatment administration is confounded with test week and wind conditions, so the faster Week 1 times cannot be attributed to caffeine alone. All athletes received caffeine in Week 1 and placebo in Week 2, meaning any systematic difference between weeks could affect results. The presence of tailwind in Week 1 versus no wind in Week 2 provides a plausible alternative explanation for the 0.7-second improvement (24.1s vs 24.8s). Proper crossover design requires counterbalancing treatment order to separate drug effects from temporal, environmental, or practice effects.
- A hydrology study measured nitrate concentration in a stream upstream and downstream of a farm. Samples were taken on three dates. Between Date 1 and Date 2, the farm applied fertilizer. Between Date 2 and Date 3, a large storm occurred.
Based on the data, which conclusion is most strongly supported?
Answer: Downstream nitrate exceeds upstream on all dates, suggesting an added nitrate source between sampling locations.
Downstream nitrate consistently exceeds upstream concentrations on all three sampling dates (2.3 vs 2.1, 3.8 vs 2.0, and 2.9 vs 2.4 mg/L), suggesting a nitrate source exists between the two sampling locations. The pattern shows the largest downstream elevation on Date 2 following fertilizer application, which decreased after the storm on Date 3 but remained above upstream levels. While the temporal association with fertilizer application is suggestive, other sources like livestock operations, septic systems, or natural groundwater inputs could also contribute to the consistently higher downstream concentrations observed across all sampling periods.
- A chemist investigated how temperature affects the rate of a reaction that produces gas. The same concentrations of reactants were used each time. The chemist measured the time required to produce 50 mL of gas at three temperatures.
Based on the data, which conclusion is most strongly supported?
Answer: Increasing temperature increases the reaction rate over 10–30°C, as shown by decreasing time to produce 50 mL gas.
Increasing temperature increases the reaction rate over the 10-30°C range, as demonstrated by the decreasing time required to produce 50 mL of gas. At 10°C, the reaction took 120 seconds, at 20°C it took 75 seconds, and at 30°C only 50 seconds. Since reaction rate is inversely related to time (faster reactions take less time), the data show that higher temperatures accelerate this gas-producing reaction. This follows the general principle that increased thermal energy enhances molecular motion and collision frequency, leading to faster chemical reactions.
- Meteorologists examined whether a city's new reflective roof policy reduced summer electricity demand. Monthly mean daytime temperature and total electricity use were recorded for June–August in the year before the policy (Year A) and the year after (Year B). No other energy policies were reported, but Year B had a cooler summer.
The authors concluded: "Reflective roofs reduced summer electricity demand."
Which statement best evaluates the validity of the author's conclusion?
Answer: Invalid: lower electricity use could be due to the cooler summer in Year B, a confounding variable.
The conclusion is invalid because the cooler summer temperatures in Year B (29.0°C vs 31.0°C) provide an alternative explanation for reduced electricity use (470 vs 520 GWh). Since air conditioning demand typically drives summer electricity consumption, the 2°C temperature difference could account for the 50 GWh reduction without any contribution from reflective roofs. The study lacks a control group and cannot separate the policy effect from the confounding variable of weather variation. Multiple factors influence electricity demand, making it impossible to attribute causation to the roof policy based on this observational data.
- Why do the measurements in Table 1 fail to support the gardener's conclusion?
Answer: No seedlings were grown without the plant food for comparison.
Seedlings grow on their own, so growing taller over 30 days is what they would do with or without the plant food. Without a group of untreated seedlings measured over the same 30 days, there is nothing to compare the treated growth against and the plant food cannot be credited. The small differences in starting height and the number of seedlings are minor next to having no comparison at all.
- A field study assessed whether a pesticide (PX) reduces aphid counts on soybean plants. Two adjacent plots were used: Plot 1 was sprayed with PX; Plot 2 was not sprayed. Aphids per plant were counted weekly for 4 weeks. During Week 3, a heavy rainstorm occurred that washed insects from leaves.
The authors concluded: "PX caused the decline in aphid numbers."
Which statement best evaluates the validity of the author's conclusion?
Answer: Invalid: aphids declined similarly in both plots, and the rainstorm provides an alternative explanation.
The conclusion is invalid because aphid populations declined similarly in both treated and untreated plots, and the heavy rainstorm in Week 3 provides a plausible alternative explanation. Both Plot 1 (PX-treated) and Plot 2 (untreated) showed nearly identical patterns: moderate declines by Week 2, sharp drops to ~10 aphids per plant during the rainstorm week, and slight recovery by Week 4. The small differences between plots (1-2 aphids per plant) are minimal compared to the dramatic decline coinciding with the weather event. The parallel trends suggest environmental factors, not pesticide treatment, drove the population changes.
- Researchers tested whether a new fertilizer (Fert-X) increases tomato yield. Twenty greenhouse benches (each bench held 10 identical tomato plants) were randomly assigned to either Fert-X or a standard fertilizer (Control). All benches received the same watering schedule and light cycle. After 8 weeks, researchers measured mean fruit mass per plant (g) for each bench and recorded greenhouse zone (East or West). The researchers noticed the West zone ran ~2°C warmer than the East zone due to a vent.
The authors concluded: "Fert-X causes higher tomato yield than the standard fertilizer."
Which statement best evaluates the validity of the author's conclusion?
Answer: Invalid: higher West-zone temperature could explain yield differences, and Fert-X shows only small within-zone increases.
The conclusion is invalid because higher temperature in the West zone provides an alternative explanation for the yield differences. The data show that West-zone benches produced higher yields (585g vs 510g for Control, 590g vs 520g for Fert-X) regardless of fertilizer type, suggesting the ~2°C temperature difference is the primary factor. Within each zone, Fert-X shows only modest increases over Control (10g in East, 5g in West), which are too small to confidently establish causation. The confounding variable of zone temperature prevents isolating the fertilizer effect from environmental factors.
- A biology class investigated whether a certain wavelength of light affects plant height. Students grew seedlings under either red LEDs or blue LEDs for 14 days. Each group used the same soil and watering schedule, but the red-LED shelf was closer to a window and received some additional sunlight.
The authors concluded: "Red light causes greater seedling growth than blue light."
Which statement best evaluates the validity of the author's conclusion?
Answer: Invalid: extra sunlight is a confounding variable, so the height difference cannot be attributed to LED color alone.
The conclusion is invalid because additional sunlight exposure is a confounding variable that prevents attributing the height difference to LED color alone. While red-LED seedlings averaged 11.4 cm compared to 9.1 cm for blue-LED seedlings, the red-LED shelf was positioned closer to a window and received extra sunlight. This uncontrolled environmental factor could fully or partially account for the observed height difference. Proper experimental design requires controlling all variables except the one being tested, so the mixed lighting conditions make it impossible to isolate the effect of LED wavelength on plant growth.
- A medical device company tested whether a new wristband reduces tremor amplitude in patients with essential tremor. Twelve patients wore the wristband for 1 hour on Day 1 (device OFF) and Day 2 (device ON). Tremor amplitude (arbitrary units) was measured during a standardized task. Day 2 measurements were always taken after Day 1.
The authors concluded: "The wristband reduces tremor amplitude."
Which statement best evaluates the validity of the author's conclusion?
Answer: Not fully supported: device ON is confounded with Day 2, so order effects could explain reduced tremor.
The conclusion is not fully supported because the device ON condition is confounded with Day 2 testing, so practice effects or day-to-day variation could explain the reduced tremor amplitude. The data show lower mean tremor on Day 2 (device ON, 6.6 units) compared to Day 1 (device OFF, 8.0 units), but since device state always corresponded to test order, the improvement could result from patient familiarity with the standardized task or natural tremor fluctuation. Proper evaluation would require counterbalancing the order of device conditions across participants to separate treatment effects from temporal confounding factors.
- A nutrition study tested whether drinking green tea lowers blood glucose after a meal. Twenty adults were randomly assigned to drink either green tea or water with an identical carbohydrate meal. Blood glucose (mg/dL) was measured at baseline and 60 minutes post-meal.
According to the data, which of the following can be concluded?
Answer: Green tea is associated with a lower mean 60-minute glucose than water, given similar baseline levels.
Green tea is associated with a lower mean 60-minute glucose level (132 mg/dL) compared to water (138 mg/dL), given that both groups had similar baseline levels (92-93 mg/dL). Both groups experienced glucose increases after the meal, which is expected following carbohydrate consumption, but the green tea group showed a smaller rise (39 mg/dL vs 46 mg/dL). This 6 mg/dL difference suggests green tea may modestly blunt the post-meal glucose response. However, the data show an association rather than proving causation, and the clinical significance of this difference would require further evaluation.
