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Combined means, variance and missing observations
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Practise the method.
Combined means, variance and missing observations
Optional extension: Combined means, variance and missing observations
Example 1
The mean of 20 values was calculated as 13, but one value was entered as 40 instead of 15. Find the corrected mean. Give an exact fraction if needed.
Show worked answer
Answer: 447
- Correct the total: 20×13−40+15.
- Divide by 20.
Example 2
Group A has 10 observations with mean 10. Group B has mean 20. Together their mean is 16. How many observations are in Group B?
Show worked answer
Answer: 15
- Let Group B have m observations.
- Solve 10+m100+20m=16.
- This gives m=15.
Example 3
The mean of 10 values was calculated as 15, but one value was entered as 32 instead of 12. Find the corrected mean. Give an exact fraction if needed.
Show worked answer
Answer: 13
- Correct the total: 10×15−32+12.
- Divide by 10.
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