Clinical practices and/or patient profiles are often heterogeneous between centers. One can therefore suspect unobserved variables differently distributed between centres (socio-economic level of patients, environmental pollution, etc.). Some of these variables may constiture confounders. Taking into account the center effect tends to adjust the results on these confounders. ​​

The center : a possible confounder

Adjustement of the results on the center

• In multivariate models (conditional approach), an adjustment on the center as an explanatory variable can be made (fixed effect). The centre can also be included as a random effect.

• Also in the conditional context, but in the particular case of a Cox model, a stratification of the baseline hazard function on the center can be performed.

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• In the context of a marginal effect, the centre can be included in the propensity score, i.e. as an explanatory variable of the logistic model predicting the probability of exposure.