What is Z score and How to Find Z score?

A Z score is a statistical measure that represents the number of standard deviations a value is from the mean of a dataset. It is used to determine how unusual or significant a particular value is within the context of the data. A 0 Z score indicates that a value is exactly average or typical for the data, while a positive Z score indicates that a value is above the mean and a negative Z score indicates that a value is below the mean. Z scores are commonly used in statistical analysis to identify outliers or extreme values in a dataset.

How to find Z score?

Z score formula:

The formula for calculating a Z score is:

Z = (X – μ) / σ


X is the data point being analyzed

μ is the mean of the population

σ is the standard deviation of the population


To find a z-score, you need to follow these steps:

  • Calculate the data set’s mean (μ) and standard deviation (σ).
  • Calculate the difference between the value you want to find the z-score for and the mean (x – μ).
  • Divide the difference by the standard deviation (step 2 divided by σ). This will give you the z-score.

For instance, the following formula would be used to determine the z-score for a value of x = 100 in a data set with a mean of 80 and a standard deviation of 10.

Mean = 80, Standard Deviation = 10

Difference between x and the mean = 100 – 80 = 20

Z-score = 20 / 10 = 2.0

This means that the value of x = 100 is 2 standard deviations above the mean.

Interpreting Z Scores

Z scores can be positive or negative, depending on whether the data point is above or below the mean. A data point with a positive Z score is above the mean; when it has a negative Z score, it is below the mean.

The larger the absolute value of the Z score, the more unusual the data point is in relation to the mean. For example, a Z score of 2.5 would be considered more uncommon than a Z score of 1.0.

Uses of Z score

Determining Data Normality

One use of the z score is to determine if a dataset follows a normal distribution. A normal distribution is a bell-shaped curve that represents the distribution of a large number of independent variables. If a dataset follows a normal distribution, most data points will be close to the mean, and the number of data points will decrease as the distance from the mean increases.

To determine if a dataset follows a normal distribution, we can calculate the z scores for each data point and plot them on a graph. The dataset likely follows a normal distribution if the data points form a bell-shaped curve. If the data points are not distributed in a bell shape, likely, the dataset does not follow a normal distribution.

Outlier Detection

Another use of the z score is to identify outliers in a dataset. Outliers are data points significantly different from the rest of the data in a dataset. They can be caused by measurement errors, data entry errors, or other factors that do not reflect the general trend of the data.

To identify outliers using the z score, we can calculate the z score for each data point and compare it to a predetermined threshold. The data point is considered an outlier if the z score is above or below this threshold. This method allows us to identify and potentially remove outliers from our dataset, as they may skew our analysis if they are not representative of the general trend of the data.

Comparing Data Across Groups

The z score can also compare data across different groups or samples. For example, if we want to reach the height of individuals in two different populations, we can calculate the z scores for the height of each individual in each population. This allows us to compare the data on a common scale and determine if there is a significant difference between the two groups.

This is especially useful when the two groups have different means and standard deviations, as it allows us to compare the data in a way that considers these differences.

Assessing the Significance of Results

The z score can also be used to determine the significance of the results in statistical analysis. In statistical hypothesis testing, we use the z score to determine whether an impact is statistically significant or could have occurred by chance.

To do this, we compare the z score of the result to a predetermined critical value. The result is considered statistically significant if the z score is above the critical value. This allows us to determine if a result is likely to be true or if it is simply due to chance.

Predictive Modeling

The z score can also be used in predictive modeling, where it can help identify patterns and trends in data that can be used to make predictions about future events. For example, a z score can be used to identify variables likely to be correlated with a specific outcome, such as the likelihood of a customer making a purchase.

By identifying these variables and their correlations with the outcome, we can build a predictive model used to make predictions about future events. This can be especially useful in business and finance, where accurate predictions can lead to better decision-making and increased profitability.

Limitations of Z Scores

There are some limitations to using Z scores, including:

  • Z scores assume that the data follows a normal distribution, which may not always be the case.
  • Z scores are sensitive to outliers, which can distort the results.
  • Z scores do not account for correlations between variables, which may affect the results.
  • Z scores do not account for the sample size of the population, which may affect the reliability of the results.


Z scores are a valuable tool for statistical analysis, providing a way to compare data points within a population and determine the probability of a data point occurring within that population. However, it is essential to consider the limitations of Z scores and use caution when interpreting the results.

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