请教HLM中fixed effect 和 random effect 的问题

.微に酸袅袅

Hi， Kenny：
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4 i2 }  H& ^& j; Cwhen 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？
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% M' n1 ^8 o' g0 Kanother 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？
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5 G+ O, }9 V9 @! sBest，
Kelly

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。