An anomalous reading is a result that does not fit the pattern of the others. You spot it by comparing repeats, or by checking a value against the trend, then you leave it out of the mean and say why. This lesson follows apparatus diagrams in practical-data interpretation, and it depends on a clear table from the results table lesson.
How do you deal with an anomaly, step by step?
- Compare repeats at the same condition. Look for one that sits far from the others.
- Check against the trend. If a value across the range breaks an otherwise steady pattern, it may be anomalous.
- Suggest a cause in words, but only as a possibility. For example, the timer was stopped late or the temperature dropped.
- Decide what to do. Repeat the test if you can. If you cannot, exclude the reading from the mean and state that you did.
- Recalculate the mean using only the remaining readings.
Worked example
Invented data. Three repeats were timed at 40 °C: 33 s, 34 s and 52 s.
Step 1, compare: 33 and 34 are close. 52 is far from both.
Step 2, trend: assume the lower and higher temperatures gave longer and shorter times, so 40 °C should lie between them. 52 s is too long compared with the other two repeats.
Step 3, possible cause: the timer may have been stopped late, or the water may have been cooler than intended.
Step 4, decision: treat 52 s as anomalous, and exclude it from the mean.
Step 5, mean: (33 + 34) ÷ 2 = 67 ÷ 2 = 33.5 s.
For comparison, including the anomaly would give (33 + 34 + 52) ÷ 3 = 119 ÷ 3 = 39.7 s (to 1 decimal place), which would pull the result away from what the good repeats show.
What mistake is easy to make?
Mistaken answer: “The 52 s reading is wrong because it is too high, so I deleted it.”
The student gave no comparison, no reason and no record of what was removed.
Saying a reading is “wrong” is not enough. A strong answer compares it with the repeats (33 s and 34 s), states it breaks the pattern, names a possible cause, and says it was excluded from the mean.
Check yourself
All data are invented.
1. Four repeat times were 45 s, 47 s, 46 s and 71 s. Which is anomalous, and what is the mean of the others?
Show answer
71 s is anomalous because it is far from the other three. Mean of the rest: (45 + 47 + 46) ÷ 3 = 138 ÷ 3 = 46 s.
2. Three titres were 22.80 cm³, 22.90 cm³ and 23.90 cm³. Which reading is anomalous, and what is the mean of the concordant titres?
Show answer
23.90 cm³ is anomalous. Mean: (22.80 + 22.90) ÷ 2 = 45.70 ÷ 2 = 22.85 cm³.
3. A class tested five concentrations and recorded these times: 0.2 mol/dm³, 120 s; 0.4 mol/dm³, 61 s; 0.6 mol/dm³, 40 s; 0.8 mol/dm³, 75 s; 1.0 mol/dm³, 24 s. Which reading looks anomalous, and what should the class do?
Show answer
75 s at 0.8 mol/dm³ breaks the pattern, because times fall steadily as concentration rises and this time is longer than the one at 0.6. The class should repeat that test, and if it cannot, say it is anomalous and do not use it when describing the trend.
Where does this lead next?
Deciding what to do with a reading is half the skill. The other half is explaining how the experiment could be made more trustworthy, which is the topic of proposing a safe conceptual improvement. The mixed practice set includes further anomaly questions, and the mistake log is useful for tracking which type of slip you repeat.
A teacher can look at how you justify an exclusion, not just the number you get. That is part of what happens in online one-to-one Chemistry tuition.