How to Calculate the Z-Value for a 98% Confidence


A z-score is a common way to standardize a set of statistical data. Statisticians, business people, scientists and market analysis use z-scores to help them find the point that a piece of data would be found in a normal distribution. A 98 percent z-value indicates very high confidence that the result is not random.

  • Use the z-score formula to solve for the 98 percent confidence. The formula is z-score = (x - mean) / sigma.

    In the formula, x is the data point, mean is the average of the numbers and sigma is the standard deviation of the numbers in your data set.

  • Find the z-value by plugging in the numbers. If x is 10, the mean is 5 and sigma is 2.3, then the z value is z = (10 - 5) / 2.3. Solving determines that z = 2.17.

  • After finding the z-value is 2.17, look up the conversion on a z-value table in a book or online. Using the table, it is learned that the z-value is 98.4996. This falls above the 98 percent confidence threshold that was set, so it is accepted. In other words, there is a 98 percent confidence that the number 10 could be found in the data.


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