How do you take and record measurements accurately in a practical?
Carrying out an investigation: choosing apparatus, taking accurate and precise measurements, repeating readings, and recording results in a table.
A concise overview of the carrying-out skills for the WJEC GCSE Science Double Award practical assessment (Unit 7), covering choosing apparatus, accuracy and precision, repeating readings and anomalies, and recording results in a clear table.
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What this dot point is asking
The WJEC Double Award practical assessment (Unit 7) and practical questions want you to carry out an investigation well: choose suitable apparatus, take accurate and precise measurements, repeat readings, spot anomalies, and record results clearly.
Choosing apparatus
Reading a scale at eye level avoids a parallax error, and using equipment that measures to smaller divisions gives more precise readings.
Accuracy and precision
Repeating readings and anomalies
Recording results
Results should be recorded in a clear table as you take them:
- column headings with the quantity and unit (for example "Temperature (degrees C)"),
- the independent variable in the first column, the dependent variable (and repeats) in the next,
- a column for the mean of the repeats,
- all readings to a consistent number of decimal places.
A clear table makes it easy to spot anomalies and to plot a graph afterwards.
Random and systematic errors
There are two main kinds of error to know. Random errors cause readings to vary above and below the true value (for example slight differences in timing); they are reduced by repeating readings and taking a mean. Systematic errors shift all the readings the same way (for example a balance not zeroed, or always reading a scale from the wrong angle); they are reduced by checking and zeroing equipment and reading scales correctly. Knowing the difference between random and systematic errors, and how to reduce each, is a common evaluation point.
Resolution and units
The resolution of an instrument is the smallest change it can measure (for example a ruler with millimetre marks has a resolution of 1 mm). Choosing an instrument with a suitable resolution gives more precise results: a stopwatch reading to 0.1 s is better for short times than one reading to whole seconds. Always record measurements with their correct units and to a consistent number of decimal places that matches the instrument's resolution. Using the right resolution and units, and not writing more decimal places than the instrument can measure, is rewarded in the practical assessment.
Try this
Q1. What does taking a mean of repeat readings reduce? [1 mark]
- Cue. The effect of random errors (improving reliability).
Q2. What should every column heading in a results table include? [1 mark]
- Cue. The quantity and its unit.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksExplain the difference between accuracy and precision, and how repeating readings improves results.Show worked answer →
A Unit 7 explain question worth 4 marks. Reward: accurate results are close to the true value (1); precise results are close to each other (have little spread) when repeated (1); repeating readings and taking a mean reduces the effect of random errors, making the result more reliable (1); repeats also let you spot anomalous results (1). Markers credit accuracy as closeness to true, precision as low spread, and the value of repeats. A common error is to use accuracy and precision to mean the same thing.
WJEC style3 marksA student gets these repeat times for a reaction: 32 s, 33 s, 48 s, 32 s. Identify the anomaly and calculate the mean of the sensible results.Show worked answer →
A Unit 7 data question. The anomaly is 48 s, which is much larger than the others (1); ignore it and find the mean of the remaining values: s (1 mark for method, 1 for the answer). Markers reward identifying the anomaly and the mean of the sensible repeats. A common error is to include the anomaly in the mean.
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