How to Calculate Confidence Intervals in Excel

This formula creates an interval with a lower bound and an upper bound, which likely contains a population parameter with a certain level of confidence:

Confidence Interval  = [lower bound, upper bound]

This tutorial explains how to calculate the following confidence intervals in Excel:

1. Confidence Interval for a Mean

2. Confidence Interval for a Difference in Means

3. Confidence Interval for a Proportion

4. Confidence Interval for a Difference in Proportions

Let’s jump in!

Example 1: Confidence Interval for a Mean

We use the following formula to calculate a confidence interval for a mean:

Confidence Interval = x  +/-  z*(s/√n)

where:

• x: sample mean
• z: the chosen z-value
• s: sample standard deviation
• n: sample size

Example: Suppose we collect a random sample of turtles with the following information:

• Sample size n = 25
• Sample mean weight x = 300
• Sample standard deviation s = 18.5

The following screenshot shows how to calculate a 95% confidence interval for the true population mean weight of turtles:

The 95% confidence interval for the true population mean weight of turtles is [292.75, 307.25].

Example 2: Confidence Interval for a Difference in Means

We use the following formula to calculate a confidence interval for a difference in population means:

Confidence interval = (x₁–x₂) +/- t*√((s_p²/n₁) + (s_p²/n₂))

where:

• x₁, x₂: sample 1 mean, sample 2 mean
• t: the t-critical value based on the confidence level and (n₁+n₂-2) degrees of freedom
• s_p²: pooled variance, calculated as ((n₁-1)s₁² + (n₂-1)s₂²) / (n₁+n₂-2)
• t: the t-critical value
• n₁, n₂: sample 1 size, sample 2 size

Example: Suppose we want to estimate the difference in mean weight between two different species of turtles, so we go out and gather a random sample of 15 turtles from each population. Here is the summary data for each sample:

Sample 1:

• x₁ = 310
• s₁ = 18.5
• n₁ = 15

Sample 2:

• x₂ = 300
• s₂ = 16.4
• n₂ = 15

The following screenshot shows how to calculate a 95% confidence interval for the true difference in population means:

The 95% confidence interval for the true difference in population means is [-3.08, 23.08].

Example 3: Confidence Interval for a Proportion

We use the following formula to calculate a confidence interval for a proportion:

Confidence Interval = p  +/-  z*√p(1-p) / n

where:

• p: sample proportion
• z: the chosen z-value
• n: sample size

Example: Suppose we want to estimate the proportion of residents in a county that are in favor of a certain law. We select a random sample of 100 residents and ask them about their stance on the law. Here are the results:

• Sample size n = 100
• Proportion in favor of law p = 0.56

The following screenshot shows how to calculate a 95% confidence interval for the true proportion of residents in the entire county who are in favor of the law:

The 95% confidence interval for the true proportion of residents in the entire county who are in favor of the law is [.463, .657].

Example 4: Confidence Interval for a Difference in Proportions

We use the following formula to calculate a confidence interval for a difference in proportions:

Confidence interval = (p₁–p₂)  +/-  z*√(p₁(1-p₁)/n₁ + p₂(1-p₂)/n₂)

where:

• p₁, p₂: sample 1 proportion, sample 2 proportion
• z: the z-critical value based on the confidence level
• n₁, n₂: sample 1 size, sample 2 size

Example: Suppose we want to estimate the difference in the proportion of residents who support a certain law in county A compared to the proportion who support the law in county B. Here is the summary data for each sample:

Sample 1:

• n₁ = 100
• p₁ = 0.62 (i.e. 62 out of 100 residents support the law)

Sample 2:

• n₂ = 100
• p₂ = 0.46 (i.e. 46 our of 100 residents support the law)

The following screenshot shows how to calculate a 95% confidence interval for the true difference in proportion of residents who support the law between the counties:

The 95% confidence interval for the true difference in proportion of residents who support the law between the counties is [.024, .296].

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