## Standard Error: Standard Deviation of Sampling Distribution

The standard error (SE) of a statistic is the standard deviation of the sampling distribution of that statistic. Standard errors reflect how much sampling fluctuation a statistic will show. The inferential statistics (deductive statistics) involved in constructing confidence intervals and significance testing are based on standard errors. Increasing the sample size, the Standard Error decreases.

In practical applications, the true value of the standard deviation of the error is unknown. As a result, the term standard error is often used to refer to an estimate of this unknown quantity.

The size of the SE is affected by two values.

- The Standard Deviation of the population affects the standard errors. The larger the population’s standard deviation ($\sigma$), the larger is SE i.e. $\frac {\sigma}{\sqrt{n}}$. If the population is homogeneous (which results in a small population standard deviation), the SE will also be small.
- The standard errors are affected by the number of observations in a sample. A large sample will result in a small SE of estimate (indicates less variability in the sample means)

**Application of Standard Errors**

The SEs are used in different statistical tests such as

- to measure the distribution of the sample means
- to build confidence intervals for means, proportions, differences between means, etc., for cases when population standard deviation is known or unknown.
- to determine the sample size
- in control charts for control limits for means
- in comparison tests such as
*z*-test,*t*-test, Analysis of Variance, - in relationship tests such as Correlation and Regression Analysis (standard error of regression), etc.

**(1) Standard Error of Means**

The SE for the mean or standard deviation of the sampling distribution of the mean measures the deviation/ variation in the sampling distribution of the sample mean, denoted by $\sigma_{\bar{x}}$ and calculated as the function of the standard deviation of the population and respective size of the sample i.e

$\sigma_{\bar{x}}=\frac{\sigma}{\sqrt{n}}$ (used when population is finite)

If the population size is infinite then ${\sigma_{\bar{x}}=\frac{\sigma}{\sqrt{n}} \times \sqrt{\frac{N-n}{N}}}$ because $\sqrt{\frac{N-n}{N}}$ tends towards 1 as *N* tends to infinity.

When the population’s standard deviation ($\sigma$) is unknown, we estimate it from the sample standard deviation. In this case SE formula is $\sigma_{\bar{x}}=\frac{S}{\sqrt{n}}$

**(2) Standard Error for Proportion**

The SE for a proportion can also be calculated in the same manner as we calculated the standard error of the mean, denoted by $\sigma_p$ and calculated as $\sigma_p=\frac{\sigma}{\sqrt{n}}\sqrt{\frac{N-n}{N}}$.

In case of finite population $\sigma_p=\frac{\sigma}{\sqrt{n}}$

in case of infinite population $\sigma=\sqrt{p(1-p)}=\sqrt{pq}$, where $p$ is the probability that an element possesses the studied trait and $q=1-p$ is the probability that it does not.

**(3) Standard Error for Difference Between Means**

The SE for the difference between two independent quantities is the square root of the sum of the squared standard errors of both quantities i.e $\sigma_{\bar{x}_1+\bar{x}_2}=\sqrt{\frac{\sigma_1^2}{n_1}+\frac{\sigma_2^2}{n_2}}$, where $\sigma_1^2$ and $\sigma_2^2$ are the respective variances of the two independent population to be compared and $n_1+n_2$ are the respective sizes of the two samples drawn from their respective populations.

**Unknown Population Variances**

Suppose the variances of the two populations are unknown. In that case, we estimate them from the two samples i.e. $\sigma_{\bar{x}_1+\bar{x}_2}=\sqrt{\frac{S_1^2}{n_1}+\frac{S_2^2}{n_2}}$, where $S_1^2$ and $S_2^2$ are the respective variances of the two samples drawn from their respective population.

** Equal Variances are assumed**

In case when it is assumed that the variance of the two populations are equal, we can estimate the value of these variances with a pooled variance $S_p^2$ calculated as a function of $S_1^2$ and $S_2^2$ i.e

\[S_p^2=\frac{(n_1-1)S_1^2+(n_2-1)S_2^2}{n_1+n_2-2}\]

\[\sigma_{\bar{x}_1}+{\bar{x}_2}=S_p \sqrt{\frac{1}{n_1}+\frac{1}{n_2}}\]

**(4) Standard Error for Difference between Proportions**

The SE of the difference between two proportions is calculated in the same way as the SE of the difference between means is calculated i.e.

\begin{eqnarray*}

\sigma_{p_1-p_2}&=&\sqrt{\sigma_{p_1}^2+\sigma_{p_2}^2}\\

&=& \sqrt{\frac{p_1(1-p_1)}{n_1}+\frac{p_2(1-p_2)}{n_2}}

\end{eqnarray*}

where $p_1$ and $p_2$ are the proportion for infinite population calculated for the two samples of sizes $n_1$ and $n_2$.