请教HLM中fixed effect 和 random effect 的问题

.微に酸袅袅

Hi， Kenny：
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: L5 z9 r' ^3 ~  nwhen we use HLM，there are some paper just report the fixed effect，while most time I use the random effect，so my question is when shall we use fixed or random effect and their difference？

8 ~) a, `2 y. d) h/ |& Aanother question is about the ground mean and the group mean， when we test the level 2 model， should we use group mean？can you give some guidance？

9 F. m! ^2 m9 v& ]4 W- xBest，
Kelly
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Kenneth

Kelly，這是個給國內同學用的問題網站，麻煩用中文提問。謝謝。