Confidence Intervals

confidence interval 4 plus or minus 2
An interval of 4 plus or minus 2

A Confidence Interval is a range of values we are fairly sure our true value lies in.

men running

Example: Average Height

We measure the heights of 40 randomly chosen men, and get a mean height of 175cm,

We also know the standard deviation of men's heights is 20cm.

The 95% Confidence Interval (we show how to calculate it later) is:

175cm ± 6.2cm

confidence interval 175 plus minus 6.2

This says the true mean of ALL men (if we could measure all their heights) is likely to be between 168.8cm and 181.2cm.

But it might not be!

The "95%" says that 95% of experiments like we just did will include the true mean, but 5% won't.

So there is a 1-in-20 chance (5%) that our Confidence Interval does NOT include the true mean.

Calculating the Confidence Interval

Step 1: find the number of observations n, calculate their mean X, and standard deviation s

Using our example:

Note: we should use the standard deviation of the entire population, but in many cases we won't know it.

We can use the standard deviation for the sample if we have enough observations (at least n=30, hopefully more).

Step 2: decide what Confidence Interval we want: 95% or 99% are common choices. Then find the "Z" value for that Confidence Interval here:

Confidence
Interval
Z
80% 1.282
85% 1.440
90% 1.645
95% 1.960
99% 2.576
99.5% 2.807
99.9% 3.291

For 95% the Z value is 1.960

Step 3: use that Z in this formula for the Confidence Interval

X  ±  Zs√n

Where:

And we have:

175 ± 1.960 × 20√40

Which is:

175cm ± 6.20cm

In other words: from 168.8cm to 181.2cm

The value after the ± is called the margin of error

The margin of error in our example is 6.20cm

confidence interval calculator

Calculator

We have a Confidence Interval Calculator to make life easier for you.

Another Example

apple tree

Example: Apple Orchard

Are the apples big enough?

There are hundreds of apples on the trees, so you randomly choose just 46 apples and get:

So let's calculate:

X  ±  Zs√n

We know:

86 ± 1.960 × 6.2√46 = 86 ± 1.79

So the true mean (of all the hundreds of apples) is likely to be between 84.21 and 87.79

True Mean

Now imagine we get to pick ALL the apples straight away, and get them ALL measured by the packing machine (this is a luxury not normally found in statistics!)

And the true mean turns out to be 84.9

Let's lay all the apples on the ground from smallest to largest:

confidence interval 86 plus minus 1.79
Each apple is a green dot,
except our observations which are blue

Our result was not exact ... it is random after all ... but the true mean is inside our confidence interval of 86 ± 1.79 (in other words 84.21 to 87.79)

But the true mean might not be inside the confidence interval but 95% of the time it will!

95% of all "95% Confidence Intervals" will include the true mean.

Maybe we had this sample, with a mean of 83.5:

confidence interval 83.5 plus minus 1.25
Each apple is a green dot,
our observations are marked purple

That does not include the true mean. Expect that to happen 5% of the time for a 95% confidence interval.

So how do we know if the sample we took is one of the "lucky" 95% or the unlucky 5%? Unless we get to measure the whole population like above we simply don't know.

This is the risk in sampling, we might have a bad sample.

Example in Research

Here is Confidence Interval used in actual research on extra exercise for older people:

confidence interval extract

What is it saying? Looking at the "Male" line we see:

"HR" is a measure of health benefit (lower is better), so that line says that the true benefit of exercise (for the wider population of men) has a 95% chance of being between 0.88 and 0.97

* Note for the curious: "HR" is used a lot in health research and means "Hazard Ratio" where lower is better, so an HR of 0.92 means the subjects were better off, and 1.03 means slightly worse off.

Standard Normal Distribution

It is all based on the idea of the Standard Normal Distribution, where the Z value is the "Z-score"

For example the Z for 95% is 1.960, and here we see the range from -1.96 to +1.96 includes 95% of all values:

confidence interval 95%
From -1.96 to +1.96 standard deviations is 95%

Applying that to our sample looks like this:

confidence interval 86 plus minus 1.79 bell
Also from -1.96 to +1.96 standard deviations, so includes 95%

Conclusion

The Confidence Interval is based on Mean and Standard Deviation. Its formula is:

X  ±  Zs√n

Where:

Confidence
Interval
Z
80% 1.282
85% 1.440
90% 1.645
95% 1.960
99% 2.576
99.5% 2.807
99.9% 3.291