Analysing and Evaluating Data » Interpreting and Evaluating Data

Interpreting data: patterns, trends and predictions

Data can come as words, diagrams, graphs, symbols or numbers. Your job is to look for a pattern or trend, then say what it means.

Follow these steps every time:

• Describe what the data shows. Use numbers from the table or graph.
• Infer what might be happening. An inference is a sensible idea that fits the data.
• Conclude. A conclusion answers the question the investigation set out to answer.

Say a student heats water in a water bath and adds a solid until no more will dissolve. The table shows how much dissolves at each temperature.

Temperature (°C)	Mass dissolved in 100 g of water (g)
20	32
40	64
60	96

The trend: as the temperature goes up, the mass dissolved goes up. It goes up by 32 g for every 20°C. So you can make a prediction: at 50°C about 80 g should dissolve.

Be careful with predictions. They are safest between your data points. Going far outside your data is a guess, because the pattern might change.

Key terms:

• Trend: the general direction the data moves, such as going up or going down.
• Inference: an idea about what is happening that is based on the data.
• Conclusion: what the data tells you in answer to the question.

Relating data to a hypothesis

Once you have data, you comment on the extent to which the data is consistent with the hypothesis. That means: how well does it fit?

Data can fit partly. Then say so: "The data mostly supports the hypothesis, but the result at one temperature does not fit the trend."

Sometimes you must pick the better of two hypotheses. Here is how:

• Write down what each hypothesis predicts.
• Check each prediction against the data.
• The hypothesis that matches more of the data, and explains it better, is the better one.

Always give the reason, using the data, not just your opinion.

Evaluating data: accuracy, precision, repeatability and reproducibility

Being objective means judging results fairly, using the evidence and not what you hoped to see. Four ideas help you do this.

Key terms:

• Accurate: a measurement that is close to the true value.
• Precise: measurements are precise if they cluster closely together.
• Repeatable: repeating the measurement, under the same conditions by the same investigator, gives similar results.
• Reproducible: similar results are obtained by different investigators with different equipment.

Picture a target. The true value is the centre.

So a set of results can be precise without being accurate. Precise results alone do not tell you whether they are close to the truth.

Errors in measurements

No measurement is perfect. Errors come in two types.

Now a different student reads a burette in a titration and gets 22.4, 22.9, 22.1 and 22.6 cm³. They vary in no set direction. That is random error, perhaps from judging the liquid level by eye. Taking more readings and finding the mean reduces it.

Anomalous values

An anomalous value is a result that does not fit the pattern of the others. Do not ignore it straight away.

• Examine it to try to find the cause.
• If it came from a poor measurement, such as spilling some liquid or stopping a timer late, you can ignore it.
• If you cannot find a cause, say that it is anomalous, but think carefully before leaving it out.

Never just remove a result because you do not like it. That is not being objective.

Suggesting improvements

When you evaluate data, you can suggest improvements to the procedure. Match the improvement to the problem:

• Results scattered (random error): take more repeats and calculate a mean.
• Results consistently off (systematic error): check or replace the equipment, for example check the thermometer against a known value.
• Results hard to read: use apparatus with a smaller scale division.

Communicating the scientific rationale

Science is only useful if people understand it. When you write a report or give a presentation, make it coherent and logically structured. A good report covers:

• The aim and the reasoning behind the investigation.
• The method you used.
• Your findings, shown in tables, graphs, numbers or symbols.
• A reasoned conclusion that links back to the hypothesis.

Use correct scientific vocabulary, units, symbols and chemical names. Write "25 cm³ of dilute hydrochloric acid", not "some acid". You can do this on paper or on a screen. You will need these skills when you evaluate the required practicals and in the exam.

Exam-style question

A student measures the mass of a solid on a balance five times. The readings are 5.02 g, 5.03 g, 5.01 g, 5.02 g and 5.03 g. The true mass of the solid is 4.50 g. The balance had not been set to zero before use.

(a) Are the results precise? Give a reason. (b) Are they accurate? Give a reason. (c) What type of error caused this? (2 + 2 + 1 marks)