stratifiyng by centre

[Irina_M]

Please, help to translate.

Phrase is used in the following sentence:
Differences between treatment groups were analysed using ... methodology stratifying by centre.

Thanks in advance

[Rikki]

методология стратификации от центра? т.е., видимо, не низ-середина-верх, а центр-окраины ...по типу как от брошенного в воду камня круги бегут