A precise limitation has three parts: the weakness, its effect on the results, and the improvement that fixes it. “Not accurate” and “human error” contain none of the three.
This page shows the pattern on an original experiment, with mistakes and corrections, and links to the data and investigation skills in biology data and investigations.
What makes a limitation precise?
Precise means a reader can picture the exact step that caused the problem. It also means the effect on the data is stated, such as “the count would be too low” or “the two variables changed together”.
Vague answers describe the experiment as a whole. Precise answers point to one step in the method.
Worked example: light and pondweed
Experiment: A student moves a lamp closer to a beaker of pondweed and counts bubbles released in one minute at each distance. The lamp distances are 10, 20, 30, 40 and 50 cm.
Limitation 1: bubbles are a poor measure of volume.
- Weakness: the bubbles differ in size, so counting them does not measure the volume of oxygen produced.
- Effect: two counts that are equal could represent different volumes of gas.
- Improvement: collect the gas in a gas syringe or an inverted measuring tube and read the volume.
Limitation 2: temperature changed with light.
- Weakness: the lamp gave off heat, so moving it closer warmed the water as well as increasing the light.
- Effect: a change in bubble rate may be partly caused by temperature, so light is not the only variable changing.
- Improvement: place a beaker of water or a heat shield between the lamp and the plant, or use a cool lamp, and check the temperature with a thermometer.
Limitation 3: one run at each distance.
- Weakness: the experiment was done once at each distance.
- Effect: an unusual result cannot be spotted, so a single odd count could distort the pattern.
- Improvement: repeat each distance at least three times and calculate a mean.
The mistakes to watch for
Mistaken answer: “Human error, so the results were not accurate.”
It names no step and no effect. It could be written about any experiment, which is why a mark scheme cannot reward it.
Mistaken answer: “The student should do it again.”
This is an improvement without a weakness. It also does not say what repeating would fix.
Correction: write the step (“the bubbles varied in size”), the consequence (“so the count does not measure volume”) and the change (“collect and measure the gas volume”).
A sentence frame
Use “The [step] meant that [effect on data], so [what cannot be trusted]. This could be improved by [specific change].”
For example: “The lamp heated the water as it moved closer, so temperature changed as well as light, and the effect of light alone cannot be known. This could be improved by placing a heat shield between the lamp and the beaker.”
Where to look for weaknesses
A quick scan of the method finds most of them.
- Measurement: is the instrument or the count a good measure of the thing you want?
- Control: did any other variable change at the same time?
- Sample size: are there enough repeats, or enough organisms?
- Timing and reading: could the start, stop or reading have been inconsistent?
Check yourself
1. Seeds are placed at 10 °C, 20 °C and 30 °C, with five seeds at each temperature. Give one limitation and an improvement.
Show answer
Only five seeds were used at each temperature, so one seed that fails for another reason would change the percentage a lot. Use a larger number, such as 20 or more, and repeat the test.
2. Leaf area is estimated by counting squares on graph paper under the leaf outline. State one limitation.
Show answer
Squares on the edge of the outline are partly covered, so deciding whether to count them gives an estimate, not an exact area. Use smaller squares or a consistent rule, such as counting squares more than half covered.
3. Why is “the thermometer was not accurate” a weak limitation?
Show answer
It does not say how the reading was affected or how the result changed. A precise version would state the direction of the error and how it would be checked, for example by comparing the thermometer with a second one in the same water bath.
Where to go next
Read how causal claims from data should be worded, then practise writing limitations with the original practice hub. The probability tree and counting board and the inheritance model board also help show why small samples can give results that differ from a model.
Online one-to-one Biology tuition gives you an assigned teacher who can take one of your own practical write-ups and help you turn each vague sentence into a precise one, while the work stays yours.