Evaluating the data

The quality of any data should be evaluated before making any conclusions.

Precision, repeatability and reproducibility

TermMeaning
Precision Measurements are in close agreement
Repeatable Measurements are very similar when repeated by the same person or group, using the same equipment and method
ReproducibleMeasurements are very similar when repeated by a different person or group, using different equipment and/or methods

Precision and repeatability can be seen easily from a table of results containing repeat measurements. If the repeat measurements are close together, the data is precise and repeatable.

Accuracy

Evaluation of the data should also consider accuracy. A measurement is accurate if it is close to the true value.

To ensure the data is as accurate as possible, work out the best estimate of the true value:

  • Identify any outliers (anomalous results) in the data. These are results that are very different to the others. For example:
2.2 and 0.1 are outliers
  • Try to explain why the outlier is different. An outlier may be removed if there is a good reason to do so. For example, if there is a measurement or recording error.
  • Find the mean of the remaining results. To find the mean add together the results and divide by the number of measurements.

Example

Using the example above:

Sum of values = 0.9 + 1.0 + 1.2 + 0.8 + 1.0 + 1.1 + 0.8 + 1.2 = 8.0

Number of measurements = 8

Mean = 8.0 ÷ 8 = 1.0

This mean is the best estimate of the true value.

The mean should be given to the same number of significant figures as the measurements in the table.

Confidence in the accuracy of results

Data cannot always be relied upon. There can be errors, and all measurements have some level of uncertainty.

Random errors are unpredictable and can be due to human error, eg in judging when to stop a timer.

Systematic errors cause results to differ from the true value by the same amount each time. These could be due to:

  • a fixed error in the measuring instrument, eg not being correctly zeroed
  • influence of the environment, eg allowing a reaction to take place at a hotter temperature
  • method of observation, eg not reading the volume of a liquid correctly using the bottom of the meniscus

To describe the accuracy of an experiment, discuss the level of confidence in the results. If only one reading was taken, then you can be less confident in the accuracy of the results.

The range describes the difference between the highest and lowest repeat results. The smaller the range, the more confident you can be in the accuracy of the result.

Evaluation of experimental strategy

Having evaluated the data, suggest improvements to the way in which the experiment was carried out that could improve the quality of the results.