International A Level Biology / Practical Skills in Biology I
Evaluating results and suggesting improvements
Edexcel International A Level Biology (YBI11), chapter 13: Practical Skills in Biology I, specification point U3.2, U3.3. Free lesson.
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Evaluating practical results means identifying what could limit a method, explaining how it affects the results and suggesting a realistic improvement. You should also check anomalous readings, calculate percentage error and draw conclusions that stay within the evidence.
Make each limitation specific to the method
A useful limitation names what goes wrong in that particular practical. “Human error” or “the results were not accurate” is too vague because it does not explain the problem or how to fix it. For colour standards, matching colours by eye is subjective. Use standards at smaller intervals or a colorimeter. For the DCPIP practical, drops can vary in size. Read the volume added from a burette instead. For beetroot discs, pigment from cut cells can add to the reading. Wash the discs until the water stays clear. For clear zones, the zones may not be perfect circles. Measure two diameters at…
Build a full evaluation
An evaluation weighs up the method rather than simply describing the data. Explain why each limitation matters, then give an improvement that could be made in the same experiment. A missing set of repeats, standard deviations or data from an investigation done only once can also limit what you can conclude.
Check an anomalous reading before finding the mean
An anomalous result is a reading that does not fit the other results. Repeat the measurement to check it. If you leave the anomaly out, calculate the mean using only the remaining readings.
Calculate percentage error from the scale uncertainty
A scale reading has an uncertainty, which describes how far the reading could be from the true value. For one reading from a scale, the uncertainty is usually half the smallest division. Percentage error compares this uncertainty with the value measured. Percentage error = uncertainty ÷ value measured × 100.
Conclude only what the data tested
A conclusion should describe the comparison the results support, without making claims about untested plants, bacteria or concentrations. A clear zone shows that bacterial growth was inhibited, not that the bacteria were killed. If garlic extract gives a larger clear zone than mint extract, conclude that garlic extract inhibited the growth of this bacterium more than mint extract did. Do not claim that garlic is the best plant for killing bacteria: only two extracts, one bacterium and one concentration were tested.
Reduce uncertainty near the breaking point
Adding 100 g masses at a time makes the recorded breaking mass uncertain. The fibre might have broken at any load up to 100 g below the mass recorded. Near the breaking point, add smaller masses, such as 10 g at a time.