How to Calculate z score of Confidence Interval (original) (raw)

Last Updated : 23 Jul, 2025

To calculate the z-score for a confidence interval, find the complement of the confidence level (1 - C), divide by 2, then use a z-table or calculator to find the z-score corresponding to the cumulative probability (1 - α/2).

z-score represents the number of standard deviations a data point is from the mean of a standard normal distribution.

Let's discuss the steps to calculate z-score of confidence interval with example for better understanding.

Steps to Calculate z-score of Confidence Interval

Here are the steps to calculate the z-score for a given confidence interval:

**Step 1: Determine the Confidence Level: Common confidence levels are 90%, 95%, and 99%. Let's denote the confidence level as C.

**Step 2: Find the Complement of the Confidence Level: Calculate α as α = 1 − C.

**Step 3: Divide the Complement by 2: This gives α/2, representing the area in each tail of the normal distribution.

**Step 4: Find the z-score: Use a z-table or a statistical calculator to find the z-score that corresponds to the cumulative probability of 1 − α/2.

Let's work through an example:

Example: 95% Confidence Interval

• **Confidence Level: 95% (or 0.95)
• **Complement: α = 1 − 0.95 = 0.05
• **Divide by 2: α/2 = 0.05/2 = 0.025
• **Find the z-score: Look up the cumulative probability of 1 − 0.025 = 0.975 in the z-table.

From the z-table, the z-score corresponding to a cumulative probability of 0.975 is approximately 1.96.

Therefore, for a 95% confidence interval, the z-score is 1.96.

Z-Scores for Common Confidence Levels

• **90% Confidence Level: α = 0.10, α/2 = 0.05, cumulative probability = 0.95, z-score ≈ 1.645
• **95% Confidence Level: α = 0.05, α/2 = 0.025, cumulative probability = 0.975, z-score ≈ 1.96
• **99% Confidence Level: α = 0.01, α/2 = 0.005, cumulative probability = 0.995, z-score ≈ 2.576

Formula Summary

To summarize, the z-score for a confidence interval C can be found using the following steps:

• α = 1−C
• α/2
• Look up the cumulative probability of 1 − α/2 in the z-score table to find the z-score.

By following these steps, you can determine the z-score corresponding to any confidence level for constructing confidence intervals.

**Read More,

• **Statistics
• **Confidence Interval

Conclusion

The Z-score for a confidence interval is calculated to understand how many standard deviations a particular value is from the mean of a data set. It is particularly useful in determining the boundaries of the confidence interval. To compute the Z-score we need the desired confidence level and corresponding Z-score which is based on the standard normal distribution. Once we have the Z-score it is used to calculate the margin of error and the interval within which the true population parameter is expected to lie.