Causal impact from such studies are thus prone to being biased due to residual confounding. Several studies caution that their findings could be due to confounding factors. The detailed accounting is difficult because of confounding factors. Bias and confounding factors cannot be excluded from the study. Hypothesized social causes are supported by only weak evidence, distorted by numerous confounding factors. Causal models offer a robust technique for identifying appropriate confounding variables. Many of these results are based on large longitudinal studies controlling for various confounding factors. Sex and age are common examples of confounding variables. This could be because of several confounding variables. Observational studies use stratification or matching as a way to control for confounding. Randomized trials are not affected by confounding by indication due to random assignment. The FDA described the potential role of confounding social stressors "unclear". In epidemiology, one type is "confounding by indication", which relates to confounding from observational studies. Neither confounding the persons nor dividing the substance. Limb, Limb, Limb and Limb did not report on looking for any confounding variables.