What is measured by the estimated standard error sM?

The estimated standard error ( s_M ) measures the variability or precision of the sample mean ( M ) as an estimate of the population mean. It quantifies how much the sample mean is expected to fluctuate from one sample to another, reflecting the degree of uncertainty associated with the sample mean due to sampling variability. A smaller ( s_M ) indicates a more reliable estimate of the population mean, while a larger ( s_M ) suggests greater uncertainty.