← Years 7–8 Data & Information

Making a prediction from a trend

Extending a trend past the known data, such as running a trendline forward, to estimate a future value. It is an estimate, not a certainty, and gets less reliable the further ahead you go.

Part of: 7-8 Digital Technologies: Analysing and visualising data — Module 25, ACARA Digital Technologies Years 7–8 — Analysing and Visualising Data — Module 48

Understanding Making a prediction from a trend in Years 7–8 Data & Information

What it does: Wording like 'predicted' and 'about' shows the number is an estimate that might turn out different. The further a trendline is extended beyond the real data, the more things can change, so the estimate becomes less trustworthy. Extending a trend past the known data gives a prediction. It is an estimate, not something that is certain to happen. Trends can change, such as a plant stopping growing. Predicting far ahead is less reliable. The Forecast option in the trendline settings extends the line forward by the number of periods you choose. A small difference is normal. A prediction is an estimate, not a certainty.

Common mistakes: FORECAST needs the known y values (sales) before the known x values (months). The ranges are swapped. It should be =FORECAST(6,B2:B6,A2:A6). The scores go up by 4 each week (12, 16, 20), so the step in the trend is 4, not 8. It should be step ← 4.

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This module's key terms are also in a printable crossword (Modules 47–80) and word search (Modules 9–80).

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Learning
Duo
A chart shows a school canteen's weekly sales rising steadily. You extend the trendline to guess next week's sales. What is this?
A guaranteeA prediction
Matches
Next month's sales will be exactly 120.
↓
A prediction is treated as a certainty. It should say the sales are predicted to be about 120.
Applying
Meanings
Term
prediction
Meaning
Extending a trend past the known data, such as running a trendline forward, to estimate a future value. It is an estimate, not a certainty, and gets less reliable the further ahead you go.
Practice
=FORECAST(11,B2:B11,A2:A11)
Testing
Gaps
=(6,B2:B6,A2:A6)
Check Answer
Spots
1A2:A6 = month, B2:B6 = sales
2=FORECAST(6,A2:A6,B2:B6)

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Quick Questions

1. A chart shows a school canteen's weekly sales rising steadily. You extend the trendline to guess next week's sales. What is this?
A. A guarantee
B. A prediction ✓
Extending a trend past the known data gives a prediction. It is an estimate, not something that is certain to happen.
2. Which statement about a prediction from a trendline is true?
A. It is an estimate that could be wrong ✓
B. It is a fact that is always right
A prediction is only an estimate. Things can change, so the real value might be different.
3. A trendline on a chart of Year 7 reading minutes goes up each week. Which is the best way to make a prediction for next week?
A. Extend the trendline forward one step ✓
B. Pick any number you like
A prediction should follow the trend. Running the trendline forward gives a sensible estimate for the next value.
4. Your chart shows 6 months of data about a phone's battery health. Which prediction is likely to be MORE reliable?
A. Predicting 5 years from now
B. Predicting next month ✓
The further ahead you predict, the less reliable the estimate gets. Next month is close to the known data.
5. Why is it risky to use 4 weeks of plant growth data to predict the plant's height in 2 years?
A. The trend may change over a long time ✓
B. Trendlines stop working after 4 weeks
Trends can change, such as a plant stopping growing. Predicting far ahead is less reliable.

Match the Pairs

Forecast forward 3 periods → Extends the trendline 3 time periods past the last data point to estimate future values
=FORECAST(13,B2:B13,A2:A13) → Predicts the value for period 13 using the pattern in the existing data
A prediction is an estimate based on a trend, not a certainty, so we say 'about' or 'around' → Predicted sales next month: about 120
Predicting far past the known data is less reliable than predicting just one step ahead → Forecast 1 period = more reliable, Forecast 50 periods = risky
Next month's sales will be exactly 120. → A prediction is treated as a certainty. It should say the sales are predicted to be about 120.
Trendline forecast: 100 periods ahead is just as accurate as 1 period ahead → Predictions get less reliable the further ahead you go, so 100 periods is far less accurate than 1.

Key Term

prediction
Extending a trend past the known data, such as running a trendline forward, to estimate a future value. It is an estimate, not a certainty, and gets less reliable the further ahead you go.

Fill in the Blanks

1. =FORECAST(6,B2:B6,A2:A6)
Hint: Type the spreadsheet function that predicts a future value from a trend.
2. Weeks 1 to 5 are in A2:A6 =FORECAST(6,B2:B6,A2:A6)
Hint: Type the week number you want to predict, the first one after the known data.
3. Trendline: y = 2x + 10 =2*6+10
Hint: Type the slope number that multiplies the week in the trendline equation.
4. Trendline: y = 2x + 10 =2*6+10
Hint: Type the operator that adds the trendline's starting value to the predicted amount.
5. A7 holds week 6 =FORECAST(A7,B2:B6,A2:A6)
Hint: Type the cell reference that holds the week you want to predict.

Spot the Error

Question 1
A A2:A6 = month, B2:B6 = sales
B =FORECAST(6,A2:A6,B2:B6)
C Result is an estimate only
Line B — FORECAST needs the known y values (sales) before the known x values (months). The ranges are swapped. It should be =FORECAST(6,B2:B6,A2:A6).
Question 2
A Scores: 12, 16, 20 (weekly)
B step ← 8
C last ← 20
D next ← last + step
Line B — The scores go up by 4 each week (12, 16, 20), so the step in the trend is 4, not 8. It should be step ← 4.
Question 3
A A2:A7 = year, B2:B7 = height
B Known years are 1 to 6
C Predict height for year 7:
D =FORECAST(6,B2:B7,A2:A7)
Line D — Year 6 is already in the known data, so this is not a prediction. To predict the next year, the first value should be 7.
Question 4
A Known data: days 1 to 10
B Day 11: close to the data
C Day 100: just as reliable
D Far predictions are riskier
Line C — The further ahead you predict, the less reliable the estimate is. Day 100 is far from the known data, so it should say much less reliable.
Question 5
A Right-click the trendline
B Choose Format Gridlines
C Forecast Forward: 2 periods
D Line extends past the data
Line B — The Forecast option is in the Format Trendline menu, not Format Gridlines. Gridlines settings cannot extend a trend forward.

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