1. How to Answer a “Source of Error” Question
A source-of-error question is not asking you to list everything that could go wrong — it wants the ONE or TWO factors that are inherent to this particular procedure/apparatus and would cause a genuine, unavoidable error in the final result. Structure every answer in two parts:
- Part 1 — What is the source of error? Be specific: name the apparatus/step and the physical reason the reading is not exact.
- Part 2 — How does it affect the result? State which quantity is affected, and whether it becomes larger than, smaller than, or larger/smaller than its true value.
Do NOT accept these as answers unless you can justify them further — examiners routinely reject them as “too general”:
- Parallax error (only acceptable if you specify why parallax cannot be avoided in this exact set-up)
- Random error / human error (on its own)
- Zero error of an instrument (this is correctable by calibration, not a key source of error)
- Anything fixed simply by “being more careful” or “repeating the reading”
1.1 Random Error vs Systematic Error
| Source of Error | Effect on the Reading / Result | Precaution / Improvement |
| Random Error | Unpredictable scatter, in either direction, caused by limits of precision or the experimenter’s inability to repeat a measurement identically. E.g. repeating a mass reading gives 17.46 g, 17.42 g, 17.44 g. | Take repeated readings and average; take more data points for a graph so the best-fit line averages out the scatter. |
| Systematic Error | A consistent bias in one direction throughout the whole experiment, usually from a fault in the apparatus or method. E.g. a stretched tape measure makes every length reading too small. | Cannot be reduced by averaging. Must identify the faulty apparatus/method and correct or recalibrate it (e.g. account for zero error, use a different instrument). |
1.2 Quick-Reference: Precision of Common Apparatus
| Apparatus | Precision (Uncertainty) | How to Read / Use Correctly |
| Metre rule / half-metre rule | 0.1 cm | Read to the smallest division; eye vertically above the scale. |
| Ammeter (0–1 A) | 0.01 A | Read to half the smallest division (0.02 A ÷ 2). |
| Voltmeter (0–3 V / 0–5 V) | 0.05 V | Read to half the smallest division. |
| Laboratory thermometer | 0.5 °C | Read to half the smallest division; last digit 0 or 5. |
| Stopwatch (digital) | 0.01 s | Record to 2 d.p. unless told otherwise. |
| Protractor | 1° | Read to the smallest division. |
| Measuring cylinder (100 ml) | 0.5 cm³ | Read at eye level, bottom of meniscus. |
| Spring balance (0–1 N) | 0.01 N | Check for zero error before use. |
| Spring balance (0–10 N) | 0.1 N | Check for zero error before use. |
| Digital micrometer / calipers | 0.01 mm / 0.01 cm | Close jaws gently; check zero error. |
| Electronic mass balance | 0.01 g | Tare (zero) before every measurement. |
2. Mechanics
2.1 Period of Oscillation (Simple Pendulum)
| Source of Error | Effect on the Reading / Result | Precaution / Improvement |
| The thread is slightly elastic, or slips through the split cork. | The effective length l of the pendulum is not what was recorded, so T is inaccurate. | Measure and re-check l just before each timing; use an inextensible thread. |
| Reaction time in starting/stopping the stopwatch is significant compared to the period of one oscillation. | T is either larger or smaller than the true value — the error is a larger fraction of the result for short single-oscillation timings. | Time a large number of oscillations (e.g. 20) and divide by 20, so the fixed reaction-time error is spread over many swings and becomes negligible per oscillation. |
| The bob does not swing in one vertical plane (it swings elliptically) or amplitude is too large. | The period recorded is not the true period for small-angle oscillation. | Release the bob with a small amplitude and check it swings in a single plane; restart if it drifts. |
| The reference point used to judge “one complete oscillation” is not fixed. | Miscounting oscillations changes T. | Use the lowest point of the swing (highest speed, easiest to judge) as the reference point for counting. |
Standard technique justification: measuring 20 oscillations and dividing by 20 divides the (constant) human reaction-time error by 20, greatly reducing its effect on the value of one period T.
The full set of notes, which includes sections such as Sources of Error by Practical Topic is available in hard copy for students who sign up for any of our regular practical lessons, Crash Courses or Mock Exams.
- Topics Tested in Past O-Level Physics Paper 3 Practical Exam
- Topics Tested in Past O-Level Chemistry Paper 3 Practical Exam
- Topics Tested in Past O-Level Biology Paper 3 Practical Exam
LOOKING FOR THE BEST PLACE TO DO YOUR SCIENCE PRACTICALS? JOIN US!
Singapore Learner has been a Comprehensive Science Practical Training provider since 2017.
As at 19 Apr 2026, we have trained about 1060 students for their science practical exams and we have conducted a total of about 4044 lab sessions.
Our laboratory apparatus are exam-grade and similar to those used in MOE schools and our chemicals are all NEA-approved.
We provide A-Level / H2 / IP and O-Level Physics, Chemistry, Biology and Combined Science (Physics/Chemistry/Biology) Practical Training/Crash Course/Mock Exams for both local (eg. H2, Singapore-Cambridge) and international exams (CIE, Pearson Edexcel, IGCSE).
Why Choose Us?
- Our teachers are very experienced, and we actually TEACH you good practical techniques.
- We have been a one-stop comprehensive science practical centre providing solid practical training for ALL THREE sciences and for all levels and streams since 2017.
- Our laboratory apparatus are exam-grade and similar to those used in MOE schools and our chemicals are all NEA-approved.
- We have a structured practical training programme catering to the needs of both beginners and experienced students.
- We have a small class size so that the teacher is able to observe the actions of each student more closely and demonstrate the correct practical techniques where and when necessary.
- Many private schools trust us to prepare and conduct science practical training and assessment for their students, including structured training, mock exams and even actual CIE science practical exams.