← ACARA Digital Technologies Years 9–10

Deciding what to do with an outlier

A value far from the rest. Check if it is a data entry error or a real extreme value, then correct, remove or keep it, and record your decision.

Part of: ACARA Digital Technologies Years 9–10 — Acquiring and Validating Data — Module 108

Understanding Deciding what to do with an outlier in ACARA Digital Technologies Years 9–10

What it does: The value was an entry error. After checking the original source, you correct it to the true value. The value is far from the rest but checking shows it is real, so you keep it rather than delete it. An outlier should be investigated first. 70 hours of sleep is impossible, so it is probably a typing error (maybe 7.0 or 7). Never delete it without checking. Outliers are only unusual values. You have to check each one to decide if it is an error or real. Recording the decision keeps your work honest and lets others check or repeat it.

Common mistakes: An age above 120 is the suspicious outlier, so the test should be age > 120. Using < would flag normal ages instead. Values far above the rest should be flagged as possible errors, so the test should be value > 250. With < the normal value 175 would be flagged.

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This module's key terms are also in a printable crossword (Modules 333–377) and word search (Modules 333–412).

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Learning
Duo
A class survey on hours of sleep shows 8, 7, 9, 8, 70. What is the first thing to do with 70?
Check whether it is a data entry errorDelete it straight away because it is big
Matches
Deleted the highest mark without checking it
↓
Removing an outlier without checking whether it is an error or real. A genuine value may be lost and results become biased.
Applying
Meanings
Term
outlier
Meaning
A value far from the rest. Check if it is a data entry error or a real extreme value, then correct, remove or keep it, and record your decision.
Practice
=IF(MAX(D2:D8)>3*AVERAGE(D2:D8),"Check outlier","Keep")
Testing
Gaps
=(B2:B9)
Check Answer
Spots
1IF age < 120 THEN
2 check the original form

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Quick Questions

1. A class survey on hours of sleep shows 8, 7, 9, 8, 70. What is the first thing to do with 70?
A. Delete it straight away because it is big
B. Check whether it is a data entry error ✓
An outlier should be investigated first. 70 hours of sleep is impossible, so it is probably a typing error (maybe 7.0 or 7). Never delete it without checking.
2. A student typed 1.65 m, 1.70 m, 1.68 m, 165 m for heights. The 165 was meant to be 1.65 m. What should you do?
A. Correct it to 1.65 and note the change ✓
B. Keep 165 because it is real data
When you know the true value, you correct the data entry error. Then record the change so others can see what you did.
3. In a dataset of 100m sprint times, one runner is much faster than the others, and the timing was checked and confirmed. What should you do?
A. Keep it because it is a genuine extreme value ✓
B. Remove it because it is far from the rest
A value can be far from the rest and still be real. If it checks out, it is genuine data, so keep it.
4. Which statement is true about outliers?
A. An outlier is always a mistake that must be deleted
B. An outlier is a value far from the rest, and it may be an error or a real value ✓
Outliers are only unusual values. You have to check each one to decide if it is an error or real.
5. A weather log shows a max temperature of 240 degrees for a Perth day, and there is no way to find the real value. What is a sensible choice?
A. Remove it, because it is an impossible error you cannot correct ✓
B. Keep it so the dataset has no gaps
If a value is clearly an error and the true value cannot be found, remove it (or mark it as missing). Keeping it would distort the results.

Match the Pairs

=MAX(B2:B9) → Finds the biggest value in a column so you can spot a possible outlier that is far above the rest
=IF(B2>100,"Check","OK") → Flags a value that is out of the possible range so you can check if it is a data entry error
A typing slip such as 810 instead of 81 is a data entry error, so correct it once you confirm the true value from the source → B9: 810 changed to 81
A genuine extreme value, such as a real 98 in a set of lower marks, is kept because it is true data → Leo Science 98: verified, keep
Deleted the highest mark without checking it → Removing an outlier without checking whether it is an error or real. A genuine value may be lost and results become biased.
Changed 98 to 80 so it fits the pattern → Changing a real value to look normal. This makes the data inauthentic. Keep a genuine extreme value and do not alter it.

Key Term

outlier
A value far from the rest. Check if it is a data entry error or a real extreme value, then correct, remove or keep it, and record your decision.

Fill in the Blanks

1. =MAX(B2:B9)
Hint: Type the function that finds the largest value in a range, which helps you spot a high outlier.
2. =IF(B2>100,"Error","OK")
Hint: Type the comparison operator that tests whether a mark is greater than the limit, because a mark over 100 is impossible.
3. =MEDIAN(B2:B9)
Hint: Type the function that returns the middle value, which is not pulled far off by an extreme value.
4. IF isError THEN correct(value) ELSE keep(value)
Hint: Type the keyword that runs when the outlier is not a data entry error but a genuine value.
5. =COUNTIF(B2:B9,">100")
Hint: Type the function that counts how many cells in a range meet a condition, so you can see how many suspect values there are.

Spot the Error

Question 1
A IF age < 120 THEN
B check the original form
C IF form says 15 THEN
D correct the value to 15
Line A — An age above 120 is the suspicious outlier, so the test should be age > 120. Using < would flag normal ages instead.
Question 2
A value ← 175
B IF value < 250 THEN
C mark as possible error
D ELSE keep the value
Line B — Values far above the rest should be flagged as possible errors, so the test should be value > 250. With < the normal value 175 would be flagged.
Question 3
A Value: 9.58 s in 100 m sprint
B Source: official world record
C Decision ← delete the value
D Log: genuine extreme value
Line C — The source confirms the time is a real world record, so it is a genuine extreme value. It should be kept, not deleted.
Question 4
A outlier ← 450 in Qty column
B Checked the original receipt
C Receipt says 45, a typo
D Log: kept 450 as genuine
Line D — The receipt shows 450 was a data entry error and the real value is 45. The log must record that it was corrected from 450 to 45.
Question 5
A Max ← MAX(B2:B9)
B Min ← MAX(B2:B9)
C IF Max > 100 THEN check it
Line B — MAX gives the largest value. To find the smallest value, and spot a low outlier, use MIN(B2:B9).

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