Confidence interval
A confidence interval is the range where the whole population's number probably falls, worked out from a sample: 52% ± 5 runs from 47% to 57%.
Select any words here to ask your Maestro about them.
A survey of 400 residents says 52% ± 5. How many do you need to ask to get the margin down to ± 2.5?
What it's for: a sample never gives exactly the whole population's number. The confidence interval tells you how far off it might be: 52% ± 5 means the real share is probably between 47% and 57%. The ± 5 is the margin of error.
How to read "95% confidence": if you ran the survey many times with other randomly chosen people, about 95 of every 100 intervals would catch the real number. It doesn't promise this particular interval has it: it's the method that's right 95 times out of 100.
An interval only covers the error of happening to pick some people and not others. If the sample is biased, the interval doesn't fix it.
For how many decimals a percentage from 40 answers deserves, see How many decimal places should a percentage have?.
How it's used
How to get it in Excel
200 delivery times: average 32 min, standard deviation 12
- marked: =CONFIDENCE.NORM(0.05,12,200)0.05 means 95% confidence; 12, the standard deviation; 200, how many values.
- 1.66The margin: the 32-minute average runs from 30.3 to 33.7.
When to look closer
A lead inside the margin
A poll of 1,000 people: A has 46% and B 43%, with a ± 3 margin. Does it say A is winning?
Ask your Maestro about Confidence interval
It answers from the free course page closest to this word, the way it answers while you study.
You: What does "Confidence interval" mean, in plain words?
Your Maestro: I'll explain it with a case of yours: it sticks better than a definition. Which poll or report you read lately came with its margin of error?
Asking about: Answers from the page "What is a row? The question that changes the number"
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Related terms
- Population and sample
The population is the whole group you want to know about; the sample is the part of it you actually measure to estimate the rest.
- Percentage
A percentage says how many parts out of every 100 something is. You get it by dividing the part by the total and multiplying by 100: 45 out of 180 customers is 45 ÷ 180 × 100 = 25%.
- Standard deviation
Standard deviation measures how far data lands from the average: with an average of 30 and an SD of 2, most values fall from 28 to 32. In Excel: =STDEV.S.
- Normal distribution (bell curve)
A normal distribution is the bell shape a lot of data takes: most values near the average, fewer and fewer out to the sides, evenly on both.