Meta-analysis has become a cornerstone of evidence synthesis across disciplines, from medical research to environmental science. Thanks to modern software, conducting a meta-analysis is more accessible than ever. However, as our recent work on the quality of meta-analyses of organochlorine pesticides highlights (https://www.nature.com/articles/s41893-025-01634-5), accessibility does not always translate to quality. A key reason is that practitioners often overlook the statistical theories underpinning meta-analysis. In a series of blog, I aim to demystify these foundational concepts, starting with a common topic: sampling variance.
What is sampling variance?
Sampling variance is a fundamental concept in statistics, yet it is often misinterpreted in meta-analysis. When researchers talk about “sampling variance,” they usually mean the variance associated with effect size estimates from primary studies. However, the term is broader: sampling variance exists whenever we estimate any parameter; an effect size, an overall mean effect, a regression coefficient, or even a variance component such as the between-study variance (tau^2). In essence, sampling variance reflects the uncertainty that arises from random sampling. It quantifies how much an estimate would fluctuate if we were to repeat the study many times under identical conditions.
Sampling variance is a fundamental concept in statistics, yet it is often misinterpreted in meta-analysis. When researchers talk about “sampling variance,” they usually mean the variance associated with effect size estimates from primary studies. However, the term is broader: sampling variance exists whenever we estimate any parameter; an effect size, an overall mean effect, a regression coefficient, or even a variance component such as the between-study variance (tau^2). In essence, sampling variance reflects the uncertainty that arises from random sampling. It quantifies how much an estimate would fluctuate if we were to repeat the study many times under identical conditions.
Source: made by the author in R 4.0.3
Sampling distribution and standard Error
To understand sampling variance, we first need to understand the sampling distribution. Imagine conducting a study with a sample size 𝑛 to estimate an effect size, such as the standardized mean difference (SMD) between treatment and control groups. If you could repeat this experiment many times (each time drawing a new random sample from the same population), you would obtain a distribution of SMD estimates. This is the sampling distribution of the SMD.
The standard error (SE) is the standard deviation of this sampling distribution. It quantifies the precision of your estimate: smaller SEs imply greater precision. The sampling variance is simply the square of the standard error (SE^2). In practice, we rarely know the true effect, so we rely on the standard error (and thus sampling variance) to gauge how much our estimate might vary due to random sampling.
In meta-analysis, sampling variance arises in several contexts:
(1) effect size estimates from primary studies,
(2) overall mean effect from an intercept-only meta-analytic model,
(3) regression coefficients from a meta-regression, and
(4) variance components (e.g., tau^2, the between-study variance).
To understand sampling variance, we first need to understand the sampling distribution. Imagine conducting a study with a sample size 𝑛 to estimate an effect size, such as the standardized mean difference (SMD) between treatment and control groups. If you could repeat this experiment many times (each time drawing a new random sample from the same population), you would obtain a distribution of SMD estimates. This is the sampling distribution of the SMD.
The standard error (SE) is the standard deviation of this sampling distribution. It quantifies the precision of your estimate: smaller SEs imply greater precision. The sampling variance is simply the square of the standard error (SE^2). In practice, we rarely know the true effect, so we rely on the standard error (and thus sampling variance) to gauge how much our estimate might vary due to random sampling.
In meta-analysis, sampling variance arises in several contexts:
(1) effect size estimates from primary studies,
(2) overall mean effect from an intercept-only meta-analytic model,
(3) regression coefficients from a meta-regression, and
(4) variance components (e.g., tau^2, the between-study variance).
Deriving sampling variance: “ideal” vs. “practical” approaches
Ideally, to calculate the sampling variance of an effect size estimate, you would repeat a study with the same sample size (n) many times, compute the effect size each time, and then calculate the standard deviation of the resulting sampling distribution.
For example, to estimate the sampling variance of an SMD, you would:
(i) Conduct the study multiple times with sample size (n).
(ii) Compute the SMD for each study.
(iii) Form the sampling distribution of SMDs.
(iv) Calculate its standard deviation (the standard error) and square it to get the sampling variance.
Similarly, for the overall mean effect in a meta-analysis, you would:
(i) Draw Randomly sample sets of studies many times.
(ii) Fit an intercept-only meta-analytic model to each sample to estimate the overall mean effect.
(iii) Form the sampling distribution of these mean effects.
(iv) Calculate its standard deviation and square it.
This conceptual exercise shows what sampling variance means, but of course, repeating studies thousands of times is impractical.
Ideally, to calculate the sampling variance of an effect size estimate, you would repeat a study with the same sample size (n) many times, compute the effect size each time, and then calculate the standard deviation of the resulting sampling distribution.
For example, to estimate the sampling variance of an SMD, you would:
(i) Conduct the study multiple times with sample size (n).
(ii) Compute the SMD for each study.
(iii) Form the sampling distribution of SMDs.
(iv) Calculate its standard deviation (the standard error) and square it to get the sampling variance.
Similarly, for the overall mean effect in a meta-analysis, you would:
(i) Draw Randomly sample sets of studies many times.
(ii) Fit an intercept-only meta-analytic model to each sample to estimate the overall mean effect.
(iii) Form the sampling distribution of these mean effects.
(iv) Calculate its standard deviation and square it.
This conceptual exercise shows what sampling variance means, but of course, repeating studies thousands of times is impractical.
From concept to calculation: statistical theory as a shortcut
Fortunately, statistical theory provides us with elegant shortcuts. Instead of repeated sampling, we rely on mathematical results that describe the expected variance of estimators under specific model assumptions. For individual effect sizes (e.g., log response ratios or Fisher’s z), Taylor series approximations can be used to derive analytical formulas for sampling variance. For overall effects and regression coefficients in meta-regression, the framework shifts to Weighted Least Squares (WLS) or, in random-effects settings, Generalized Least Squares (GLS) estimation.
Fortunately, statistical theory provides us with elegant shortcuts. Instead of repeated sampling, we rely on mathematical results that describe the expected variance of estimators under specific model assumptions. For individual effect sizes (e.g., log response ratios or Fisher’s z), Taylor series approximations can be used to derive analytical formulas for sampling variance. For overall effects and regression coefficients in meta-regression, the framework shifts to Weighted Least Squares (WLS) or, in random-effects settings, Generalized Least Squares (GLS) estimation.
These results rest on classic statistical theorems like Gauss–Markov theorem and Minimum Variance Unbiased Estimator (MVUE). Thus, the familiar SE formulas in meta-analysis are not arbitrary; they are rooted in the same optimality principles that underpin regression theory. Understanding this connection not only clarifies what the standard error represents but also why it behaves as it does, linking the practical mechanics of meta-analysis back to its statistical roots.