An anomalous result is a reading that does not fit the pattern of the rest of the data. In an SPM answer you identify it, suggest a cause, and say what you did with it.
This lesson is part of interpreting enzymes and experimental rates. It follows choosing a fair comparison.
What is the three-question check?
Before you call a reading anomalous, ask these questions in order.
- Does the reading differ clearly from its own repeats at the same condition?
- Does it break the trend shown by the other conditions?
- Can you name a likely cause in the method?
If all three answers are yes, you can treat the reading as anomalous. If not, it may be a real feature of the data, such as an enzyme optimum.
Worked example: a table with repeats
An original experiment measures the volume of oxygen collected from catalase in 2 minutes at three temperatures. Each is repeated three times.
| Temperature | Trial 1 (cm³) | Trial 2 (cm³) | Trial 3 (cm³) |
|---|---|---|---|
| 20 °C | 8 | 9 | 8 |
| 30 °C | 14 | 15 | 27 |
| 40 °C | 20 | 21 | 20 |
Question 1: Identify the anomalous result and explain why.
The third trial at 30 °C, 27 cm³. It is far from its repeats (14 and 15) and it is higher than the readings at 40 °C, which breaks the rising trend.
Question 2: Suggest one cause.
The gas may have been collected for longer than 2 minutes, or extra enzyme extract may have gone into that tube. Either would give a larger volume.
Question 3: Calculate the mean at 30 °C with and without the anomaly.
With all three: (14 + 15 + 27) ÷ 3 = 56 ÷ 3 = 18.7 cm³. Without the anomaly: (14 + 15) ÷ 2 = 14.5 cm³.
The mean with the anomaly, 18.7 cm³, is pulled upward by one value. The mean without it, 14.5 cm³, fits the trend between 8.3 at 20 °C and 20.3 at 40 °C. If the question allows, report both and explain which you trust.
Should you ever keep an odd result?
Keep the value in your table and mention it. A cautious line in a conclusion reads: “One reading at 30 °C (27 cm³) was not used in the mean because it differs from its repeats, and a timing error is a likely cause.”
If the odd value is the only reading at a condition, with no repeats, you cannot tell whether it is an error. The right response is to say that the result should be repeated before it is trusted.
The mistake that costs marks
The common error is to delete the value and say nothing, or to call any awkward reading an anomaly. Both lose the reasoning mark.
| Student wrote | Problem | Better version |
|---|---|---|
| “27 is wrong so I removed it” | No evidence or cause | “27 differs from its repeats 14 and 15; a timing error is likely” |
| “The 50 °C result is an anomaly” | It may be denaturation | “The fall at 50 °C fits the enzyme being denatured, so it is not anomalous” |
| “Average all three anyway” | Hides the problem | “I report the mean with and without the anomaly” |
Note that a drop in rate above the optimum temperature is expected, so it is not an anomaly.
Check yourself
Rates for the same enzyme at pH 7 are 10, 11 and 4 units in three repeats. Identify the likely anomaly, and calculate the mean without it.
Answer
The value 4 is the likely anomaly. It is far from the other two readings, 10 and 11.
Mean without it: (10 + 11) ÷ 2 = 10.5 units. A cause could be a tube contaminated with acid, which would lower the pH and slow the enzyme. The experiment should be repeated to confirm.
What to study next
Use this in a full conclusion in writing a qualified conclusion from a small classroom dataset. The biological data interpretation trainer has more tables with odd values to practise on.
If you want a teacher to give you tables with unusual values and talk through them, see online one-to-one Biology tuition or the one-hour trial class (from RM50).