In Monte Carlo based importance sampling estimations. Effective Sample Size (ESS) is an important index of simulation efficiency. since ESS can measure the divergence between the target distribution and the proposal distribution effectively. and thus is widely used to decide whether resampling is needed or not. https://www.markbroyard.com/save-big-quest-mv40-gas-floor-fryer-40-lb-casters-natural-gas-discount-best-mega/