ScienceCBSE Class 8 · Exploring the Investigative World of Science

Fair Testing and Systematic Investigation: Class 8 Science

Updated 17 October 2026 · Fact-checked

A fair test changes only one factor at a time and keeps all other conditions the same, so any difference in the result can be put down to that factor. Systematic investigation means asking a focused question, planning the trials, keeping notes of everything observed, and then using the results to ask better questions.

Understand Fair Testing and Systematic Investigation

Once you have a testable question and know what you can change and observe (the previous topic), the next step is to do the experiment in a way that you can trust. The textbook says it is better to change only one thing at a time while keeping the other conditions the same.

Why one change at a time? Suppose you fry a thick puri in cold oil and a thin puri in hot oil. The thick one does not puff but the thin one does. You cannot say whether thickness or temperature was responsible, because both differed. If you want to study the effect of oil temperature, take circles of dough of the same thickness, drop them in the same way, and change only the oil temperature, for example boiling hot, hot and not very hot oil. Now a difference in puffing can be linked to temperature.

Three kinds of factors in a fair test. The factor you choose to change is the one under study. The things you keep the same (thickness, flour, way of dropping) are the controls. The result you read, such as whether it puffs or the time in seconds, is what you observe or measure.

Keep notes of everything. Record what you see and sense: did the oil splatter, smell or smoke? Notes are not always about the main answer, but they can reveal something unexpected.

Use results to ask more questions. After one round of experiments you may ask: do puris puff better when made fresh or from stored dough? What happens if I prick a hole in the puri before frying? Each answer feeds the next question. The textbook calls this the idea of systematic investigation, and says all experiments, from the simplest to the most complicated, follow the same pattern.

Observation first, then creative thinking. The preface of the book uses two symbols: the root, for staying grounded in careful observation, and the kite, for letting ideas soar. A good investigation uses both: honest observations and imaginative explanations that are then tested.

In school questions, you are usually given a messy plan and asked to find which condition was not kept the same, or you are asked to design a fair plan of your own.

Fair Testing and Systematic Investigation: formulas and key facts

Rule of a fair test: Change ONE factor
Keep all other factors the same
Observe the result; If two factors change together, the cause of the difference cannot be known.
Steps of systematic investigation: Ask a question
Plan trials
Do the experiment
Record notes
Explain
Ask a new question; The cycle repeats; each result leads to new questions.
Puri example: fair test of oil temperature: Same thickness, size, flour, way of dropping
Changed oil temperature (boiling hot, hot, not very hot)
Observed puffing and time; Based on the textbook's own example.

Same	thickness, size, flour, way of dropping
Changed	oil temperature (boiling hot, hot, not very hot)
Observed	puffing and time

How to solve Fair Testing and Systematic Investigation questions

Use this method to check or design a fair test.

1Write down the question and the one factor to be changed.
2List every other factor that could affect the result.
3Decide how each of them will be kept the same (same size, same time, same amount).
4State what is observed or measured and with what instrument.
5Plan to repeat the trial and record notes of smell, sound and unexpected things.
6After the results, write the conclusion only for the factor you changed, then write one new question.

Fair Testing and Systematic Investigation in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Spotting an unfair test

Suppose Rohan wants to know whether soaking pulses reduces their cooking time. He soaks 100 g of moong for 4 hours and cooks it in 300 mL of water on a high flame. He cooks 150 g of unsoaked moong in 250 mL of water on a low flame.

(a) List the factors that differ besides soaking. (b) Why is the comparison unfair? (c) Rewrite the plan so that it is fair. (d) If the soaked pulses take 18 minutes and the unsoaked take 30 minutes in a fair repeat, how much time does soaking save as a percentage of the unsoaked time?

Try it first, then show the solutionSolution

Differences: amount of pulses (100 g and 150 g), volume of water (300 mL and 250 mL) and flame (high and low).
Many factors changed together, so any difference in time cannot be linked to soaking alone.
Fair plan: use 100 g of moong each, 300 mL water each, the same flame, and change only soaking (4 hours or none).
Time saved = 30 − 18 = 12 minutes; percentage = 12 ÷ 30 × 100 = 40%.

Answer: (a) Mass, water volume, flame; (b) several factors change together; (c) same mass, water and flame, only soaking differs; (d) 40%.

Memory hook: One switch at a time: flip one, freeze the rest.

Repeat trials and averaging

Imagine a class drops equal circles of dough into oil of the same temperature three times and measures the time to puff: 12 s, 14 s and 13 s.

(a) Find the mean time. (b) Another group, with the same oil, gets 12 s, 13 s and 21 s. Find their mean and say which reading looks doubtful. (c) What should the group do about it?

Try it first, then show the solutionSolution

(a) Mean = (12 + 14 + 13) ÷ 3 = 39 ÷ 3 = 13 s.
(b) Mean = (12 + 13 + 21) ÷ 3 = 46 ÷ 3 ≈ 15.3 s. The 21 s reading is far from the others.
(c) Repeat that trial, look for a difference in the way the dough was put in, and keep notes about it.

Answer: (a) 13 s; (b) about 15.3 s, and 21 s looks doubtful; (c) repeat and check what differed.

Memory hook: Repeat to be sure; an odd one out is a question, not an answer.

Tricks to remember Fair Testing and Systematic Investigation

Short memory aids. Say each one aloud once, then use it on a question the same day.

Flip one switch
Imagine a panel of switches. A fair test flips only one switch and leaves the others as they were.
Use it when: Checking whether a plan is a fair test.
Root and kite
Root = careful observation and notes; kite = imaginative ideas. Both are needed.
Use it when: Recalling the message of the chapter preface.
SAME list
Before every trial write a SAME list: what stays the same? Then a CHANGE list with exactly one item.
Use it when: Planning answers.

Fair Testing and Systematic Investigation: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Why change only one factor · 2 marks · Short answerBoard level

Explain in two or three sentences why a fair test changes only one factor at a time. Use the example of frying a puri at three oil temperatures.

Try it first, then show the solutionSolution

If two or more factors change together, we cannot tell which one caused the difference in the result. (1)
For the puri, use dough circles of the same thickness, same flour, dropped in the same way, and vary only the oil temperature; then any difference in puffing is due to temperature. (1)

Answer: One change at a time lets us link the result to that factor; for puris, keep thickness, flour and dropping the same and vary only the oil temperature.

Example 2: Pick the fair plan · 1 mark · MCQBoard level

A student wants to know whether the size of a paper plane's wings changes the distance it flies. Which plan is fair?
(a) Big wings made of thick card, thrown hard; small wings made of thin paper, thrown softly.
(b) Big wings and small wings, both of the same paper, thrown with the same effort from the same spot.
(c) Big wings thrown indoors; small wings thrown outdoors.
(d) Big wings thrown once; small wings thrown ten times.

Try it first, then show the solutionSolution

(a), (c) and (d) change more than wing size (material and throwing effort; place; number of throws).
(b) changes wing size alone and keeps paper, effort and starting spot the same.

Answer: (b)

Example 3: Find the flaws and fix the plan · 5 marks · Long answerHarder board level

A student tests whether more sunlight makes a plant grow faster. Plant A is placed on a sunny window and given 100 mL of water daily in a large pot. Plant B is a different kind of plant, kept in a dark cupboard in a small pot and given 50 mL of water every other day.
(a) List the differences other than sunlight. (2)
(b) Write a fair plan. (2)
(c) Name what will be measured and its unit. (1)

Try it first, then show the solutionSolution

(a) Kind of plant, pot size, amount of water and how often it was given. (2)
(b) Use two plants of the same kind and age in identical pots with the same soil; give both 100 mL of water daily; keep one in sunlight and one in a place with much less light. Only light differs. (2)
(c) Height of the plant in cm (measured with a ruler at the same time each week); the number of leaves can also be counted. (1)

Answer: (a) Plant type, pot size, water volume and frequency; (b) identical plants and pots, same water, only the light differs; (c) height in cm.

Example 4: Reading results and deciding the next question · 4 marks · Case-basedHarder board level

Read the passage and answer.

A class fries equal circles of dough in oil at three temperatures. In boiling hot oil the dough puffs in 8 s, in hot oil in 15 s and in not very hot oil it does not puff in 60 s. Notes: the boiling hot oil splattered and smoked; the dough in the cooler oil became oily.
(a) Name the factor changed and the factors that must have been kept the same. (1)
(b) How much longer did the hot oil take to puff than the boiling hot oil? (1)
(c) State what the results suggest about oil temperature and puffing. (1)
(d) Write one new question suggested by the notes. (1)

Try it first, then show the solutionSolution

(a) Oil temperature was changed; thickness, size, flour and way of dropping were kept the same. (1)
(b) 15 − 8 = 7 seconds longer. (1)
(c) Hotter oil puffed the dough faster, and in the not very hot oil it did not puff in the time tested. (1)
(d) Any sensible question, for example: Does very hot oil that smokes change how the puri tastes or looks? Does the dough soak more oil in cooler oil? (1)

Answer: (a) Changed: oil temperature; same: thickness, size, flour, dropping; (b) 7 s; (c) hotter oil puffs the dough faster; (d) e.g. does cooler oil make the puri oilier?

Example 5: Spot the hidden unfairness · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

Two friends test whether stirring makes sugar dissolve faster. Sunita adds 5 g of sugar to 100 mL of water at room temperature and stirs; it dissolves in 40 s. Tarun adds 5 g of sugar to 100 mL of warm water and does not stir; it dissolves in 55 s. Sunita says: 'Stirring saves 15 seconds.'
(a) Which factor did the friends overlook? (1)
(b) Why is Sunita's statement not supported? (1)
(c) Suggest a corrected plan with one changed factor. (1)
(d) If the corrected trial at room temperature without stirring takes 90 s, how many times longer is it than with stirring (40 s)? (1)

Try it first, then show the solutionSolution

(a) The temperature of the water differed (room temperature versus warm) as well as stirring. (1)
(b) Stirring and temperature both differed, so the 15 s difference cannot be put down to stirring. In fact, warm water alone may help dissolving, so the effect of stirring could be even larger. (1)
(c) Use 5 g of sugar and 100 mL of water at the same temperature in both; stir one and do not stir the other. (1)
(d) 90 ÷ 40 = 2.25 times longer. (1)

Answer: (a) Water temperature; (b) two factors changed; (c) same temperature, change only stirring; (d) 2.25 times.

Where students lose marks in Fair Testing and Systematic Investigation

The slips examiners see most often, and what to write instead.

Mistake: Naming the factor changed but not what is kept the same.
Students think the fair test is complete once the variable is named.
Wrong working
Change the oil temperature and see what happens.
Right working
Change the oil temperature; keep thickness, flour and way of dropping the same; measure time to puff.
Do this: Always add a 'kept the same' line with at least two items.
Usually costs: 1 to 2 marks.
Mistake: Concluding too much from one trial.
A single result looks convincing.
Wrong working
One puri did not puff in cool oil, so cool oil never lets a puri puff.
Right working
In this trial the puri did not puff in cool oil; repeat the trial to be sure.
Do this: Say 'in this trial' and suggest repeating.
Usually costs: 1 mark.
Mistake: Changing two factors to save time.
Combining changes seems faster.
Wrong working
Use thin dough in hot oil and thick dough in cold oil.
Right working
Use same thickness; vary only the oil temperature.
Do this: Flip one switch only.
Usually costs: 2 marks in a plan.
Mistake: Averaging with an unexplained odd reading and not saying so.
Students avoid discussing errors.
Wrong working
Mean of 10, 11, 19 s = 13.3 s, reported without comment.
Right working
Mean 13.3 s, but the 19 s reading is far from the rest; it should be repeated and the reason noted.
Do this: Comment on any value far from the others.
Usually costs: 1 mark.

Exam tips

In 'is this test fair?' questions, list every factor that differs, then say which one was meant to differ.
When asked to design a test, use the pattern: Question, Change, Keep the same, Measure (with unit), Repeat, Notes.
Use the words 'because' and 'so' to link the single change to the result.
Mention that extra observations (smell, smoke, splatter) are written down.
Show a mean with the working: add the readings, divide by how many there are.

Practice questions on Fair Testing and Systematic Investigation

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 3 marks · Short answerBoard level
Name the three kinds of factors in a fair test: the one changed, the ones kept the same and the one measured. Give an example of each using the puri.
Hint
Oil temperature, thickness and time to puff are possible choices.
Answer
Changed: oil temperature (1). Kept the same: thickness of dough, flour, way of dropping (1). Measured: time to puff, or whether it puffs (1).
Q2 · 1 mark · MCQBoard level
A student wants to test whether the mass of a ball changes how high it bounces. Which of the following must be kept the same?
(a) Height from which it is dropped
(b) Mass of the ball
(c) Both (a) and the type of floor
(d) None of these
Hint
Mass is the factor being changed.
Answer
(c) Both the drop height and the floor must be kept the same; mass is the factor changed.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): In a fair test, the results of two trials with different oil temperatures can be compared only if the dough is the same in both.
Reason (R): Differences in dough would change the result in addition to the temperature, making the cause unclear.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Does R explain why dough must be the same?
Answer
(a) R correctly explains A.
Q4 · 4 marks · Short answerHarder board level
A group wants to find whether the thickness of a rolled dough circle changes the time a puri takes to puff. (a) Name the factor changed. (b) List four factors to be kept the same. (c) Name the quantity measured with its unit. (d) Why should each thickness be tried at least three times?
Hint
Think of the other things in the chapter's list of controllable factors.
Answer
(a) Thickness of the dough circle (1). (b) Any four: oil temperature, type of flour, size (diameter) of the circle, way of dropping into the oil, amount of oil (1). (c) Time taken to puff in seconds (1). (d) One trial may be affected by chance or small differences, so repeats show whether the result is steady (1).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Three trials with the same oil temperature gave puffing times of 10 s, 11 s and 19 s for dough of equal thickness. A student writes: 'The average is 13.3 s, so a puri takes 13.3 s.'
(a) Check the average. (b) Which reading looks doubtful, and what should be done? (c) Find the average of the two close readings. (d) Which average better describes the usual time, and why?
Hint
Add the readings carefully, then divide by the number of readings.
Answer
(a) (10 + 11 + 19) ÷ 3 = 40 ÷ 3 ≈ 13.3 s, so the arithmetic is right. (b) 19 s is far from the others; repeat that trial and note differences. (c) (10 + 11) ÷ 2 = 10.5 s. (d) 10.5 s is likely closer to the usual time if the 19 s trial had a known fault, but only after repeating it can it be dropped (1 mark each).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What does a fair test do?Show answer
Changes only one factor at a time and keeps all other conditions the same.
2.Why is a fair test needed?Show answer
So that the change in the result can be linked to the one factor that was changed.
3.Give an example from the chapter of keeping conditions the same.Show answer
Use circles of dough of the same thickness, dropped in the same way, when testing different oil temperatures.
4.What follow-up questions did the chapter suggest?Show answer
Do puris puff better when fresh or from stored dough? What happens if a hole is pricked before frying?
5.What do the root and the kite on the pages stand for?Show answer
The root stands for grounded, careful observation; the kite stands for creative thinking.

Fair Testing and Systematic Investigation: frequently asked questions

What is a controlled factor?

It is a condition you keep the same in every trial, such as the thickness of the dough when you vary the oil temperature, so that it cannot cause the difference in the result.

Why repeat an experiment?

A single trial can be affected by chance. Repeating it shows whether the result is steady and helps you spot an odd reading.

Does the investigation end with the answer?

No. According to the chapter, one round of experiments usually leads to more questions, which start the next round.

Can an unusual observation be useful?

Yes. Notes on things like splattering or smell may point to a new question even when they do not answer the main one.