because the observational quantity contains information about the correlation between "X" and "Z", and the interventional quantity does not (since "X" is not correlated with "Z" in a randomized experiment). The statistician desires the unbiased estimate formula_1, but in cases where only observational data are available, an unbiased estimate can only be obtained by "adjusting" for all confounding factors, namely, conditioning on their various values and averaging the result. In the case of a single confounder "Z", this leads to the "adjustment formula":