7.7 - Evaluating a Method - Errors, Anomalies & Reliability
Evaluation questions
- Evaluation questions ask you to judge how well an experimental method worked.
- The most common tasks are:
- Identify a source of error and pair it with an improvement that reduces it.
- Spot an anomalous result and say what to do with it.
- Explain why the investigation is repeated or why a mean is taken.
- Explain why a measurement is taken in more than one place or over multiple swings.
A typical question
The student timed the trolley over a short distance using a stopwatch.
(i) Identify a source of error in this investigation.
(ii) Suggest an improvement that would reduce this error.
Sources of error and improvements
- A source of error is something in the method that makes a reading less accurate.
- An error must be paired with an improvement that reduces it - naming an error on its own doesn't score.
Common errors and their improvements
| Source of error | Why it affects the result | Improvement that fixes it |
|---|---|---|
| Timing a short event by hand with a stopwatch | Reaction time makes up a large share of a short reading | Time twenty swings and divide by twenty, or use light gates and a timer |
| Reading a scale from an angle | The value looks too high or too low due to parallax | Put your eye level with the scale and read it straight on |
| Hot water losing energy to the room during a run | The temperature drop includes energy lost to the surroundings | Insulate the container and fit a lid |
| Holding a syringe or apparatus in your hand | Warmth from your hand raises the temperature inside | Clamp the apparatus in a stand instead of holding it |
Stating "repeat the experiment" or "be more careful" does not count as an improvement for a specific measurement error. Repeating improves reliability, not accuracy.
Anomalies, reliability and validity
- An anomaly is a result that doesn't fit the pattern of the others.
- When you spot an anomaly, leave it out when calculating the mean so it doesn't drag the average off.
Repeating and taking a mean = reliability
- Reliability means getting consistent values when the experiment is repeated.
- Repeat the measurement and calculate a mean because:
- It allows anomalies to be spotted and removed.
- The mean of several repeats sits closer to the true value than any single reading.
Reliability vs validity
- Reliability is improved by taking repeats and calculating a mean.
- Validity is ensured by keeping control variables constant to maintain a fair test.
Measuring in more than one place
When an object isn't completely uniform (such as the thickness of a wire), measure at several points along it and at different angles, then take a mean so the value represents the whole object.
Common exam questions
Question 1
A student times a single swing of a pendulum with a stopwatch. Give one source of error and an improvement that would reduce it.
The swing is short, so the student's reaction time makes up a large share of the reading; time twenty swings and divide by twenty, so the same reaction-time error is spread across all of them.
Question 2
Why does "the stopwatch is a source of error" on its own not score?
Because an error only counts when it is paired with a matching improvement; you also need to say what you would change, such as timing many swings or using light gates.
Question 3
A student holds a syringe of gas in their hand while taking readings. Identify the problem and an improvement.
The warmth of the hand raises the temperature of the gas, so the temperature is not being held constant; clamp the syringe in a stand instead of holding it.
Question 4
One of five readings is far higher than the other four. What is it called, and what should you do with it when finding the mean?
It is an anomaly, and you leave it out of the mean, calculating the average from the four consistent readings.
Question 5
Describe how the reliability of a set of results could be improved.
Repeat the measurement and take a mean at each value of the variable you changed, so that consistent values across the repeats show the result is reliable.
Question 6
A question asks why the other variables were kept the same. Why is "to make the results reliable" the wrong answer?
Because keeping the other variables the same is what makes the test fair, so the answer is about validity, not reliability; reliability comes from repeating the measurements and averaging.
Question 7
A student measures the diameter of a wire at one point only. Why is measuring at several points better?
Because the wire may not be exactly the same thickness all the way along, so one measurement may not represent the whole wire; several measurements and a mean give a figure that does.