Cambridge Lower Secondary CheckpointStage 7

Thinking and Working Scientifically: Analysis, evaluation and conclusions

Science Stage 7 Chapter Notes

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Thinking and Working Scientifically: Analysis, evaluation and conclusions
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In this chapter

  1. 1Analysing Data: Identifying Patterns and Trends
  2. 2Identifying and Handling Anomalous Results
  3. 3Drawing Valid Conclusions
  4. 4Evaluating Your Method: Reliability and Validity
  5. 5Improving the Experiment: Actionable Suggestions

2. Identifying and Handling Anomalous Results

An anomalous result, or outlier, is a data point that does not fit the overall pattern of the other results. These can occur due to a mistake during the measurement (e.g., misreading a scale), a sudden change in conditions (e.g., a gust of wind), or simply random chance. It is crucial not to ignore them. When you spot an anomaly in a table of repeat readings, you should first circle or highlight it. When calculating a mean (average) from your repeats, you should exclude the anomalous result, but you must state that you have done so and why. On a graph, anomalous points will be far away from the line or curve of best fit. You should draw your line of best fit ignoring the anomaly, but circle the point on the graph to show you have identified it.

Mean = (Sum of non-anomalous values) / (Number of non-anomalous values)

Key term

Anomalous Result: A result that does not fit the general pattern of the other measurements and is considered to be an error.

Examiner insight

Marks are available for both identifying the anomaly and for correctly recalculating the mean from the remaining valid results.

Common pitfall

Students often just cross out an anomalous result without explaining that they have identified it as an anomaly and excluded it from their calculation of the mean.

Worked example 13 marks

A student measured the time taken for a reaction to finish at a certain temperature. They repeated the experiment four times and recorded their results: 52 s, 54 s, 71 s, 53 s. Calculate the mean time for the reaction.

  1. 1

    Step 1: Identify any anomalous results. The results are 52, 54, 71, and 53. The value of 71 s is much higher than the others, which are all close together. This is an anomaly.

  2. 2

    Step 2: State which result is anomalous and will be excluded from the calculation. 'The result 71 s is anomalous and will be ignored.'

  3. 3

    Step 3: Calculate the mean using the remaining, concordant results. Mean = (52 + 54 + 53) / 3.

  4. 4

    Step 4: Complete the calculation. Mean = 159 / 3 = 53 s. State the final answer with units.

  5. 5

    Final Answer: The result 71 s is an anomaly. The mean of the other results is (52 + 54 + 53) / 3 = 53 s.

Recap

  • Anomalies are results that do not fit the pattern.
  • Identify anomalies in tables and on graphs.
  • Do not include anomalies when calculating a mean.
  • State that you have ignored an anomaly and why.
  • When drawing a line of best fit, ignore the anomaly but circle it to show you've seen it.

Quick check

  1. A set of repeat measurements are 10.1, 10.2, 10.0, 12.1, 10.3. Which is the anomalous result?1 mark

3. Drawing Valid Conclusions

A conclusion is a short, powerful statement that summarises the key findings of your investigation and relates them back to your original aim or hypothesis. Your conclusion must be based only on the evidence you have collected. A good conclusion will:

  1. State clearly whether your results support or contradict your initial hypothesis.
  2. Describe the relationship or pattern you found, using the language of your analysis (e.g., 'As X increases, Y decreases').
  3. Briefly support this statement with evidence from your results, such as quoting the overall change observed.

Key term

Conclusion: A judgement or decision reached by reasoning, based on the evidence gathered in an investigation.

Examiner insight

Top-level answers explicitly state whether the hypothesis is supported or refuted. Simply stating the trend is not a full conclusion.

Common pitfall

Confusing a conclusion with a summary of what was done. A conclusion is an interpretation of the results, not a description of them.

Fun fact

The process of drawing a conclusion from evidence is a form of inductive reasoning, where specific observations are used to make a broader generalization. This is the opposite of deduction, which starts with a general rule.

Worked example 13 marks

A student's hypothesis was: 'The greater the concentration of acid, the faster the rate of reaction'. They collected results showing that 0.5 M acid produced 10 cm³ of gas in 60s, while 1.0 M acid produced 10 cm³ of gas in 30s. Write a conclusion for this experiment.

  1. 1

    Step 1: Relate the findings back to the hypothesis. The results show that a higher concentration leads to a faster reaction, which supports the hypothesis.

  2. 2

    Step 2: State the conclusion clearly. 'My results support the hypothesis.'

  3. 3

    Step 3: Add the pattern observed. 'The investigation shows that as the concentration of the acid increases, the rate of reaction also increases.'

  4. 4

    Step 4: Use data to summarise the evidence. 'For example, doubling the concentration from 0.5 M to 1.0 M halved the time taken for the reaction to complete, indicating a faster rate.'

  5. 5

    Final Answer: The results support the hypothesis. As the concentration of the acid increases, the rate of reaction increases. This is shown by the time taken to produce 10 cm³ of gas decreasing from 60s to 30s when the concentration was doubled from 0.5 M to 1.0 M.

Recap

  • A conclusion must link directly back to the initial hypothesis or aim.
  • State whether your evidence supports or contradicts the hypothesis.
  • Use your analysis of the pattern in your data to form the conclusion.
  • Do not introduce new ideas or explanations in the conclusion.
  • Keep the conclusion concise and based strictly on the results obtained.

Quick check

  1. What is the difference between results and a conclusion?2 marks

4. Evaluating Your Method: Reliability and Validity

Evaluation is about critically assessing your experiment. This involves considering two key ideas: reliability and validity.

Reliability is about the consistency and repeatability of your results. If you did the experiment again, would you get the same results? Reliability is improved by taking multiple repeat readings at each data point and calculating a mean. If your repeat readings are close together (concordant), your data is reliable.

Validity is about whether your experiment was a 'fair test' and actually measured what you set out to measure. To ensure validity, you must identify and control all other variables that could affect your dependent variable. For example, in an enzyme experiment looking at temperature, you must control the pH and substrate concentration. If a key variable is not controlled, the experiment is not valid, and your conclusion may be incorrect.

Key term

Validity: The extent to which an investigation is a fair test that measures what it is intended to measure, by controlling all other relevant variables.

Examiner insight

Examiners reward students who can clearly distinguish between reliability and validity. Linking a weakness in the method directly to its effect on the results is a key skill.

Common pitfall

Stating 'human error' as an evaluation point. You must be specific, e.g., 'difficulty in starting and stopping the stopwatch at the exact same time'.

Worked example 13 marks

In an experiment to find how light intensity affects the rate of photosynthesis in pondweed, a student counts bubbles. They use a desk lamp and move it closer to the beaker. Identify one variable that has not been controlled and explain why this reduces the validity of the experiment.

  1. 1

    Step 1: Identify a potential uncontrolled variable. The desk lamp produces heat as well as light.

  2. 2

    Step 2: Explain how this variable could change during the experiment. As the lamp is moved closer to the beaker, the temperature of the water will increase.

  3. 3

    Step 3: Explain why this affects the dependent variable. Temperature also affects the rate of photosynthesis (it affects enzyme activity).

  4. 4

    Step 4: Link this to validity. The student cannot be sure if the change in bubble rate is due to the light intensity (the intended independent variable) or the temperature (the uncontrolled variable). This makes the test unfair and reduces the validity of the conclusion.

  5. 5

    Final Answer: The temperature of the water is not controlled. The lamp produces heat, so as it moves closer, the water will get warmer. Since temperature also affects the rate of photosynthesis, we cannot be certain that the change in bubble rate is solely due to light intensity. This makes the experiment invalid.

Recap

  • Reliability is about consistency and is improved by taking repeat measurements.
  • Validity is about fairness and is ensured by controlling all other variables.
  • A source of error is anything that could have caused inaccurate results.
  • Systematic errors affect all readings in the same way (e.g., a miscalibrated thermometer).
  • Random errors cause scattered readings (e.g., judging a colour change by eye).

Quick check

  1. To make results more reliable, what should you do?1 mark
  2. What is the name for a variable you keep the same to ensure a valid test?1 mark

5. Improving the Experiment: Actionable Suggestions

A crucial part of scientific work is reflecting on your method and suggesting how it could be improved. For every weakness or limitation you identify in your evaluation, you should propose a specific, practical improvement. Crucially, you must also justify your suggestion by explaining how it would lead to more reliable or valid results. Vague suggestions like 'be more careful' or 'use better equipment' will not get marks. Your suggestions should be concrete and actionable.

Key term

Justification: The action of showing something to be right or reasonable, for example explaining how an improvement increases the validity of an experiment.

Examiner insight

Proposing the use of technology like sensors, dataloggers, or video recording to measure time more accurately are excellent, high-level suggestions for improvement.

Common pitfall

Suggesting an improvement without explaining *why* it is an improvement. The justification is where most of the marks are.

Worked example 13 marks

A student measured the rate of a reaction by judging when a cross drawn on paper disappeared below a cloudy solution. In their evaluation, they noted this was subjective. Suggest an improvement to this method and justify it.

  1. 1

    Step 1: Identify the weakness. The weakness is the subjective judgement of when the cross disappears.

  2. 2

    Step 2: Propose a specific, instrumental improvement. 'Instead of judging by eye, place the beaker on a light sensor (or LDR) connected to a datalogger.'

  3. 3

    Step 3: Justify the improvement by explaining how it works and what it improves. 'The light sensor would measure the amount of light passing through the solution. The reaction could be stopped when the light reading falls to a specific, pre-determined value.'

  4. 4

    Step 4: State whether it improves reliability or validity. 'This would remove human judgement and reaction time error, making the measurement of the endpoint consistent for every trial and therefore making the results more reliable and accurate.'

  5. 5

    Final Answer: An improvement would be to place the flask on a light sensor connected to a datalogger. The reaction could be timed until the light level detected drops to a set value. This removes subjective human judgement about when the cross disappears and provides a more precise and reliable endpoint.

Worked example 22 marks

In the photosynthesis experiment from the previous subtopic, the student did not control the temperature. Suggest an improvement to control this variable.

  1. 1

    Step 1: State the improvement. 'Place the beaker containing the pondweed into a larger beaker of water (a water bath).'

  2. 2

    Step 2: Explain how this controls the variable. 'The water in the outer beaker absorbs most of the heat from the lamp, keeping the temperature of the water with the pondweed constant.'

  3. 3

    Step 3: State what should also be done. 'The temperature should be monitored with a thermometer to ensure it does not change.'

  4. 4

    Step 4: Justify the improvement. 'This ensures that only light intensity is affecting the rate of photosynthesis, making the experiment a valid test.'

Recap

  • For every weakness identified, suggest a specific improvement.
  • Avoid vague suggestions like 'do more repeats' without saying why.
  • Justify your improvement by explaining how it increases reliability or validity.
  • Suggestions for using equipment like dataloggers or water baths are often good points.
  • Think about how to control variables more precisely or measure outcomes more accurately.

Quick check

  1. Why is 'use a measuring cylinder instead of a beaker to measure volume' a good suggestion for an improvement?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. A graph shows that as catalyst concentration increases from 0 to 2 g/dm³, the rate of reaction increases, but from 2 to 4 g/dm³ the rate stays constant. Describe the trend shown in the graph.3 marks
  2. A student records the following repeat readings for the length of a spring: 15.2 cm, 15.1 cm, 15.3 cm, 16.2 cm. Identify the anomalous result and calculate the correct mean length. Show your working.3 marks
  3. A student's hypothesis is 'slugs prefer dark, damp conditions'. They place 10 slugs in the centre of a four-chambered choice box (light/dry, light/damp, dark/dry, dark/damp). After 30 minutes, 8 slugs are in the dark/damp chamber. Write a conclusion for this investigation.2 marks
  4. In an experiment investigating how the extension of a spring depends on the mass added, state the independent variable, the dependent variable, and one variable that must be controlled.3 marks
  5. A student investigates cooling rates. They measure the temperature of hot water in two beakers, one silver and one black. They conclude 'The black beaker is a better emitter of heat'. Criticise this conclusion.2 marks
  6. Explain the difference between reliability and validity, using the context of an experiment to measure the effect of pH on amylase activity.4 marks
  7. A student measures the time it takes for magnesium ribbon to react in different acid concentrations. They use a stopwatch and their own judgement to decide when the fizzing has stopped. Identify two weaknesses in this method and suggest a specific, justified improvement for each.4 marks
  8. A student's results for an experiment are plotted on a graph. One point is clearly anomalous. Should the student join all the dots, or draw a line of best fit? Explain your answer.2 marks
  9. A student suggests that to improve an experiment, they should 'do more repeats'. Explain why this improves reliability but does not necessarily make the experiment more valid.3 marks
  10. In an experiment to see if surface area affects the rate of a reaction, a student reacts a single large marble chip with acid and then the same mass of powdered marble chip with acid. They do not measure the temperature of the acid. Explain why this is a limitation and how it could affect their conclusion.4 marks

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