Students grow identical bean seedlings for four weeks with 0, 2, 4, or 6 g of fertilizer per pot. They measure final height. What is the independent variable?

A team places water fleas in tanks with different road-salt concentrations and records how many survive after 48 hours. What is the dependent variable?

Researchers build model watersheds with 10%, 30%, or 50% paved surface, then measure runoff after equal rainfall. What is the independent variable?

A farmer tests whether cover-crop density reduces erosion by planting plots at three densities and weighing soil collected downhill. What is the dependent variable?

A pond study exposes identical algae cultures to 6, 10, or 14 hours of light daily and measures chlorophyll concentration. What is the independent variable?

At sites 10, 50, and 200 m from a highway, students measure particulate concentration using the same sampler. What is the independent variable?

Compost bins are maintained at 15 °C, 25 °C, or 35 °C, and students measure the mass lost from identical food scraps. What is the dependent variable?

Ecologists fence some grassland plots, allow light grazing in others, and allow heavy grazing in others. They count native plant species after one year. What is the independent variable?

Aquarium tanks receive 0, 5, or 10 fish of the same species per 100 L, and turbidity is measured after two weeks. What is the dependent variable?

Engineers compare three biosand filter materials by passing equal volumes of muddy water through each and measuring effluent turbidity. What is the independent variable?

A class tests whether activated carbon reduces a pesticide concentration in water. Identical samples are either passed through carbon or left untreated. What role do the untreated samples serve?

Students test a biodegradable detergent by washing equal oil-coated tiles with plain water, a weak detergent solution, or a strong detergent solution. Which condition is the control?

To test whether shade trees lower stream temperature, researchers compare shaded and unshaded reaches at the same time of day. Which factor should they keep constant?

A team tests four phosphorus doses on duckweed. Each dose is applied to six identical containers. Why are the six containers per dose useful?

A study tests whether a wetland plant removes nitrate from water. All tanks receive the same nitrate concentration except one set has no plants. What is the no-plant set?

Students compare soil percolation beneath lawns with and without compost. Which variable should remain the same in every trial?

Researchers test a low-flow showerhead by measuring water used during identical 5-minute showers with the new and standard showerheads. What is the control treatment?

A class studies acid rain effects by watering seedlings with solutions of different pH. Which condition is most appropriate as a control?

A researcher measures algae in a fertilized pond in July and an unfertilized pond in October, then attributes the difference to fertilizer. What is the main flaw?

A student sprays herbicide on one invasive-plant plot and leaves one other plot untreated. What design flaw most limits the conclusion?

To compare air quality near two factories, students sample one site at 8 a.m. and the other at 5 p.m. What is the major confounding variable?

A group tests water filters, but uses a new turbidity meter for filtered water and an older meter for unfiltered water. What flaw is introduced?

Students investigate whether a recycling campaign changes household waste by surveying only volunteers at the campaign kickoff. What is the main problem?

A class tests whether nitrate raises algal growth, but every high-nitrate tank is placed by a sunny window and every low-nitrate tank is placed in shade. What flaw prevents a clear conclusion?

A researcher estimates plastic litter at a beach by counting only items larger than 10 cm, then reports the count as total plastic litter. What is the flaw?

A team tests a water-saving irrigation system on a tomato field during a rainy month and a conventional system on the same field during a dry month. What is the main flaw?

Students test whether a pollutant affects mayfly abundance by sampling downstream after a spill but never sampling upstream. What is missing?

A scientist compares solar-panel output on a clean panel in January with a dusty panel in July. What prevents attribution of any difference to dust?

A class tests whether compost improves radish growth, but gives compost pots 100 mL of water daily and no-compost pots 200 mL daily. What flaw is present?

Investigators count birds at restored wetlands only on calm mornings but count birds at unrestored wetlands only on windy afternoons. What is the design flaw?

In 12 matched stream reaches, six receive riparian tree planting and six do not. After two summers, planted reaches have lower peak water temperatures. What does this result support?

Researchers find that counties with more public-transit riders have lower per-capita carbon dioxide emissions, but they did not assign transit use. What can they conclude?

A lab randomly assigns identical soil columns to receive either fertilizer runoff or plain water. Columns receiving runoff show more nitrate in the effluent. What conclusion is supported?

After a city installs LED streetlights, nighttime electricity use falls 18%, but the city also closes several municipal buildings. What can the data establish?

Four replicate tanks at each of three salinities show that higher salinity is followed by lower seed germination. What does replication add to the conclusion?

Students measure dissolved oxygen before and after a wastewater treatment upgrade at one stream, while also measuring an upstream reference site. Downstream oxygen rises while upstream oxygen stays similar. What is supported?

A survey finds homes that use rain barrels have lower municipal water bills than other homes. What cannot be concluded without assigning rain barrels randomly?

A field experiment applies mulch to randomly chosen garden plots and finds less soil moisture loss than in unmulched plots. Which conclusion fits the evidence?

A stream-monitoring team records lower turbidity after a construction site adds silt fences, but rainfall was much lower during the later sampling period. What is warranted?

A model predicts that replacing pavement with permeable pavement will reduce runoff. A pilot test on three parking lots finds lower runoff after replacement. What does the pilot most directly provide?