Adventures in Machine Learning

Beyond the Mean: Understanding Standard Error of the Mean

The Standard Error of the Mean: A Crucial Statistical Measure

Statisticians use several measures to determine the central tendency of a dataset, including the Mean and Median. Determining the central tendency, however, is just part of the equation.

The Standard Error of the Mean (SEM) is another critical measure for understanding a dataset’s characteristics. SEM measures the average variability of the mean of sample data sets from the population mean.

The use of SEM is especially important when working with large datasets, which can often be difficult to analyze. This article will explain how to calculate SEM using two different methods and interpret its results.

Method 1: Using SciPy Stats Library to Calculate SEM

The SciPy Stats library is a Python module that comes with an array of statistical functions, including sem(). To use this method, one needs to install SciPy, a process that is easy to perform.

To calculate SEM, we need to start with an array of values. The Python code is as follows:


import scipy.stats as stats

data = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100]

sem_result = stats.sem(data)


In this example, our sample size is 10.

The output of sem_result is 8. Method 2: Using NumPy to Calculate SEM

The second approach uses NumPy to calculate the SEM.

Numpy is a powerful library for carrying out scientific computations and data manipulation. We can calculate SEM using the following Python code:


import numpy as np

data = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100]

sample_std = np.std(data, ddof=1)

n = len(data)

sem = sample_std / np.sqrt(n)


In this example, ddof=1 specifies that we are calculating the sample standard deviation, rather than the population standard deviation. The output of sem is 8, in line with the results obtained using SciPy.

Interpreting the Standard Error of the Mean

The Standard Error of the Mean shows how much data is spread out around the mean. The smaller the SEM, the closer the data is to the mean.

A larger SEM suggests that the data is more dispersed around the mean. Furthermore, sample size is a crucial factor that can affect the SEM.

As the sample size increases, the SEM decreases, which suggests that the data is clustering around the mean. To understand this, consider two different samples: Sample A and Sample B.

Sample A consists of 20 data points, while Sample B consists of 100 data points. Suppose that both samples have the same mean, 50.

If we calculate the SEM for both samples using either of the methods outlined earlier, we will get two different results. Sample A:


data = [40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78]

sem_a = stats.sem(data)


The output of sem_a is 2.67.

Sample B:


data = [25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79, 81, 83, 85, 87, 89, 91, 93, 95, 97, 99]

sem_b = stats.sem(data)


The output of sem_b is 1.76. The results show that Sample B has a smaller SEM than Sample A, even though the data points have the same mean.

This difference is due to the sample size: Sample B has a larger sample size, which leads to a smaller SEM value.


The Standard Error of the Mean is an essential statistical measure that provides valuable information about the variability of a dataset around its mean. While there are different methods for calculating SEM, the two methods explored here using SciPy’s sem() function and NumPy’s std() function, are both reliable.

Interpreting SEM results requires consideration of sample size, where the smaller the SEM, the closer the data points are clustered around the mean. As such, SEM provides a valuable tool for making informed conclusions about a dataset.

In summary, the Standard Error of the Mean (SEM) measures the average variability of the mean of a sample dataset from the population mean. This statistical measure is vital for understanding a dataset’s characteristics, particularly when dealing with large datasets.

It can be calculated using two methods: SciPy and NumPy. The smaller the SEM, the closer the data points are clustered around the mean, and sample size plays a crucial factor in interpreting SEM results. Thus, SEM provides valuable insights to make informed conclusions and is an essential tool in statistical analyses.

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