← ACARA Digital Technologies Years 7–8

Faulty sensor readings and outliers

A reading far from the rest or impossible, like -999. It may mean the sensor is faulty or in the wrong place, so check it before you use it.

Part of: ACARA Digital Technologies Years 7–8 — Acquiring and Storing Data — Module 24

Understanding Faulty sensor readings and outliers in ACARA Digital Technologies Years 7–8

What it does: COUNTBLANK counts the empty cells in the readings column, so it shows how many readings are missing because the sensor stopped. If the maximum is much bigger than the other readings, it may be an outlier from a faulty sensor, so check it before you use it. Moisture cannot go below 0%. An impossible value like this is a sign of a possible fault. -999 is far from the others and impossible as a temperature. The other list changes only slightly. It might be a real event or a fault. Check first so you do not throw away good data or keep bad data.

Common mistakes: A single = assigns a value instead of comparing. To test for the impossible -999 reading, use == (or ===): if (temp == -999). input.temperature is a function, so it needs brackets to run and give a reading. It should be input.temperature().

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Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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This module's key terms are also in a printable crossword (Modules 204–236) and word search (Modules 189–236).

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Learning
Duo
A classroom temperature sensor logs 21, 22, 22, 23, then -999. What should you do with the -999?
Check it first, because it is impossible and may be a sensor faultKeep it and include it in the average
Matches
Average temperature = 18, 19, -999, 20 -> use all four readings as they are
↓
The -999 reading is impossible and was not checked, so it will drag the average far too low
Applying
Meanings
Term
outlier
Meaning
A reading far from the rest or impossible, like -999. It may mean the sensor is faulty or in the wrong place, so check it before you use it.
Practice
=IF(OR(MAX(B2:B8)>50,MIN(B2:B8)<-10),"Check sensor","Readings OK")
Testing
Gaps
=(B2:B30)
Check Answer
Spots
1let temp = input.temperature()
2if (temp = -999) {

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Quick Questions

1. A classroom temperature sensor logs 21, 22, 22, 23, then -999. What should you do with the -999?
A. Keep it and include it in the average
B. Check it first, because it is impossible and may be a sensor fault ✓
-999 is an impossible room temperature. It is often a code a faulty sensor sends, so check it before using it.
2. Which reading is most likely an outlier in this list of room temperatures: 20, 21, 21, 22, 85?
A. 21
B. 85 ✓
85 is far from the rest and too hot for a classroom, so it may be a fault. Check it before you use it.
3. A temperature sensor is placed next to a heater and reads much higher than the other sensors. What could this mean?
A. The sensor may be in the wrong place ✓
B. The whole school is that hot
A sensor in the wrong place, like next to a heater, can give readings far from the rest. Check where it is.
4. A sensor logs a reading every minute, but nothing is recorded from 10:05 to 10:20. What is the best explanation to check first?
A. The sensor may have stopped working for a while ✓
B. The temperature disappeared for 15 minutes
Gaps in the data can mean the sensor stopped, lost power or lost connection. Check the sensor before using the data.
5. Which statement about outliers is correct?
A. An outlier may be a fault, so check it before you use it ✓
B. An outlier is always correct, so never question it
Outliers can be real or faulty. You need to check before you decide to keep, fix or remove them.

Match the Pairs

-999 → An impossible temperature reading that usually means the sensor has failed, so check it before use
=IF(OR(B2<-10,B2>50),"Check","OK") → Flags a temperature reading that is impossible or far outside the expected range for checking
A sensor stops working for a while, leaving gaps in the log where readings should be → =COUNTBLANK(B2:B50)
Find the highest reading to spot a value that is far from the rest → =MAX(B2:B50)
Average temperature = 18, 19, -999, 20 -> use all four readings as they are → The -999 reading is impossible and was not checked, so it will drag the average far too low
A temperature sensor sits in direct sunlight on a hot car roof, so 58 is taken as the air temperature → The sensor is in the wrong place, so its reading is far from the true air temperature and cannot be trusted

Key Term

outlier
A reading far from the rest or impossible, like -999. It may mean the sensor is faulty or in the wrong place, so check it before you use it.

Fill in the Blanks

1. =MIN(B2:B30)
Hint: Type the function that finds the smallest reading, which would reveal an impossible low value.
2. =IF(B2=-999,"Check","OK")
Hint: Type the impossible number a sensor often records when it fails.
3. =IF(OR(B2<-10,B2>50),"Check","OK")
Hint: Type the function that is true when either condition is true.
4. =COUNTBLANK(B2:B30)
Hint: Type the function that counts empty cells, showing gaps when a sensor stopped.
5. =MAX(B2:B30)
Hint: Type the function that finds the highest reading, to spot a sensor placed near a heater.

Spot the Error

Question 1
A let temp = input.temperature()
B if (temp = -999) {
C console.log("Check sensor")
D }
Line B — A single = assigns a value instead of comparing. To test for the impossible -999 reading, use == (or ===): if (temp == -999).
Question 2
A let temp = input.temperature
B if (temp < -50 || temp > 60) {
C console.log("Faulty")
D }
Line A — input.temperature is a function, so it needs brackets to run and give a reading. It should be input.temperature().
Question 3
A let r = [21, 22, -999, 23]
B let good = []
C for (let x of r {
D if (x > -999) good.push(x)
E }
Line C — The closing bracket is missing from the for loop heading. It should be for (let x of r) { so the loop keeps only readings that are not the -999 fault value.
Question 4
A let avg = 21
B let reading = 95
C let diff = reading + avg
D let isOutlier = diff > 20
Line C — To see how far a reading is from the rest, subtract the average from it. Adding them gives a meaningless number. It should be reading - avg.
Question 5
A Lowest: =MIN(B2:B20)
B Highest: =MAX(B2:B20)
C Average: =AVERAGE(B2:B20)
D Gaps: =COUNT(B2:B20)
Line D — COUNT only counts cells that hold numbers, so it misses the gaps made when a sensor stops. COUNTBLANK counts the empty cells.

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