Implementation: criticising data and spotting inconsistent readings

Checking units and values

If a question asks whether the units in a table are appropriate, calculate something and compare it with the table. For example, a 2.50 V p.d. across a component of about 200 Ω gives I = 2.50 / 200 = 0.0125 A = 12.5 mA. If the table says 12.5 under 'I / A', the unit should be mA. A calculation that is not compared with the table does not answer the question.

Inconsistent readings

• A point well away from the line of best fit is an inconsistent reading. Name it (its values), check it has been plotted correctly, and repeat the measurement.
• Inconsistent decimal places or significant figures within a column are a separate criticism.
• Do not discard data just because it is inconvenient.

Is the data enough?

To judge whether data can test a prediction, look for:

• too few readings to plot a reliable graph;
• too limited a range of the independent variable (name it);
• no repeats, if the uncertainty matters to the conclusion.

Comments about intervals or missing means do not answer 'is the data sufficient to test the prediction?'.

Circuits in experiments

Know why each component is there. In a diode experiment, a variable resistor may be adjusted to keep the p.d. across the diode constant while the temperature changes; a fixed resistor already limits the current. Explain why the p.d. might drift otherwise (for example because the diode's resistance changes with temperature).

Worked example

A table lists V / V = 2.50 and I / A = 12.5 for a resistor of about 200 Ω. Are the units appropriate?

1. Expected current: I = V / R = 2.50 / 200 = 0.0125 A
2. 0.0125 A = 12.5 mA
3. The table value matches 12.5 mA, not 12.5 A.

Answer: No: the current column should be headed I / mA

Check yourself

1. A component of resistance about 150 Ω has 3.00 V across it. What current should be recorded, in mA?
   Show answer

   20 mA. I = 3.00 / 150 = 0.020 A = 20 mA.

2. A table headed 'I / A' contains 20.1, 19.8 and 20.3 for currents through a 150 Ω resistor with 3.0 V across it. What should the heading be?
   • I / A
   • I / kA
   • I / μA
   • I / mA
   Show answer

   I / mA. 3.0 / 150 = 0.020 A = 20 mA.

3. On a graph, one point lies well off a line that fits all the others. What should the student do?
   • Draw the line through it
   • Check its plotting, then repeat that measurement
   • Ignore it without comment
   • Start a new experiment
   Show answer

   Check its plotting, then repeat that measurement. Identify, check and repeat inconsistent readings.

4. A student tests a prediction that resistance depends on temperature using readings at 20 °C, 22 °C and 24 °C. What is the main weakness?
   • The temperatures are in °C, not K
   • There are no units
   • Too few readings over too narrow a range of temperature
   • The intervals are equal
   Show answer

   Too few readings over too narrow a range of temperature. Three readings over 4 °C cannot establish a trend.

5. In an experiment on a diode at different temperatures, why is a variable resistor adjusted between readings?
   • To keep the p.d. across the diode constant
   • To limit the current
   • To measure the temperature
   • To increase the sensitivity of the voltmeter
   Show answer

   To keep the p.d. across the diode constant. A fixed resistor limits current; the variable resistor keeps the controlled p.d. constant as the diode's resistance changes.

6. A column of times reads 12.4, 12.65, 12.5, 13 (s). Comment on it.
   Show answer

   The decimal places are inconsistent: all times should be recorded to the same number of decimal places (the resolution of the timer), for example 12.40, 12.65, 12.50, 13.00 s.. Consistent decimal places within a column.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics specification (XPH11/YPH11), Issue 3
• Pearson qualifications: International Advanced Levels Physics (2018)
• Pearson examiners' report: IAL Physics WPH16/01, October 2024
• Pearson examiners' report: IAL Physics WPH16/01, June 2023

Last updated 8 November 2026. Spotted a mistake? Tell us.