Variables in an experiment

Correct answer: B. It defines a dependent variable

The outcome obtained or measured at the end is the dependent variable. The independent variable is manipulated by the investigator.

Correct answer: A. The catalyst became a second influential independent factor

A catalyst can also change reaction rate so two factors changed together. Change only one independent variable at a time.

Correct answer: D. It allows easier comparison of groups with different sample sizes

Percentage accounts for differences in total seed number. Proportions are more useful when comparing unequal groups.

Correct answer: C. Change in length

Change in length accounts for differences in initial size. Choose a dependent variable that clearly represents the actual effect.

Correct answer: B. The effect may appear smaller than measurement uncertainty

A change produced by very small differences may be hidden by measurement error. Choose a sufficiently wide and safe range of values.

Correct answer: D. Effects cannot be separated because two independent factors change together

Both temperature and container size can affect evaporation. Only one factor should be changed in one experiment.

Correct answer: A. Change in pulse rate can be compared for each person

The actual change can be found by comparing final and initial values. A baseline measurement reduces the effect of individual differences.

Correct answer: C. A controlled variable is a kept-constant factor while a control group is a baseline comparison group

A controlled variable is kept constant in every trial while a control group provides a comparison baseline. Do not confuse the two terms.

Correct answer: B. To reduce the effect of random variation through replication

Using several samples under the same condition makes the average more reliable. Replication reduces the effect of random variation.

Correct answer: D. Rain-induced surface activity of earthworms

Rain may bring earthworms closer to the surface and alter the count. Keep sampling conditions consistent and representative.

Correct answer: C. Plant height will be measured in centimetres after seven days

A clear method including time and unit provides an operational definition of the dependent variable. Use measurable values instead of vague terms.

Correct answer: A. By measuring light intensity with a light meter

Distance is an indirect indicator of light intensity while a light meter gives the actual value. Measure the independent variable directly when possible.

Correct answer: B. A systematic trend or bias

A gradually changing controlled factor can shift results in one direction. Check constant conditions throughout the experiment.

Correct answer: A. Pressure also affects gas volume

Volume can change with pressure even without a temperature change. Keep pressure controlled to isolate the effect of temperature.

Correct answer: D. Give both groups a test with the same questions and difficulty

Different test papers can affect achievement scores and act as a confounding variable. Keep measurement conditions identical for both groups.

Correct answer: C. It can take many numerical values within a range

The answer is C. A continuous independent variable is a factor that can be changed through a range and can take many numerical values, often including decimal values. For example, temperature may be 20°C, 20.5°C, or 20.55°C, and time may be 2 s or 2.4 s. A, only two names, describes neither continuity nor numerical variation. B, no value, is opposite to the meaning of a variable; it must have a value. C is correct because many values are possible within a range. D, always controlled, confuses independent with controlled: the independent variable is changed, while controlled variables are kept the same. Exam cue: continuous means measurable on a scale, not merely two categories; examples include length, mass, temperature, and time.

Correct answer: B. Categorical variable

Light colours are distinct categories rather than continuously ordered numerical values. Therefore it is a categorical independent variable.

Correct answer: A. Strength of the magnet

Changing magnet strength can also change the attraction force. The same magnet should be used in every trial.

Correct answer: C. Take repeated measurements at each value and calculate the mean

Repeated trials help detect random error and allow a mean to be calculated. Perform sufficient trials at each value.

Correct answer: B. Maintain equal light intensity in all chambers

Light also affects growth so it should remain equal. This makes the independent effect of temperature clear.

Correct answer: D. In practice some variables can only be maintained within an acceptable range

Real experiments may have small fluctuations in factors such as temperature. They should be monitored and kept within an acceptable range.

Correct answer: C. Plant height changed and fertilizer amount measured

Plant height is normally measured as an outcome while fertilizer amount is changed. Their roles are reversed in the third option.

Correct answer: A. Total mass of limestone

If total mass differs the amount of available substance will also affect the result. Only particle size should change.

Correct answer: B. Randomise the order of trials

Random order prevents time-related external effects from being linked to one temperature. Vary trial order when order effects are possible.

Correct answer: D. Results suggest an effect of fertilizer but uncontrolled soil acidity may limit the conclusion

A scientific conclusion should match the evidence and acknowledge limitations. Do not claim definite causation when a variable was uncontrolled.