Table 7.3. Required School Sample Sizes to Detect Target Effect Sizes, for Various Random Assignment (RA) Designs

List of Tables

Assumes a Two-Tailed Test, a Value of .15 for the Intraclass Correlations, and a Balanced Allocation of the Research Groups
Unit of Treatment Assignment: Other Fixed or Random Effects	Number of Schools Required to Detect an Impact in Standard Deviation Units of:
Unit of Treatment Assignment: Other Fixed or Random Effects	.20	.25	0.33
I: Students Within Sites (Schools or Districts): Site Effects Fixed
R² = 0	14	9	5
R² = .2	11	7	4
R² = .5	7	5	3
R² = .7	4	3	2
II: Classrooms Within Schools: Fixed School Effects
R² = 0	51	33	19
R² = .2	41	26	15
R² = .5	26	16	9
R² = .7	15	10	6
III: Schools: Within Districts: No Random Classroom Effects
R² = 0	130	83	48
R² = .2	104	66	38
R² = .5	65	42	24
R² = .7	39	25	14
IV: Students Within Schools: Random Site Effects
R² = 0	42	27	15
R² = .2	33	21	12
R² = .5	21	13	8
R² = .7	12	8	5
V: Classrooms Within Schools: Random School Effects
R² = 0	79	50	29
R² = .2	63	40	23
R² = .5	39	25	14
R² = .7	24	15	9
VI: Schools Within Districts: Random Classroom Effects
R² = 0	167	107	61
R² = .2	134	85	49
R² = .5	83	53	31
R² = .7	50	32	18
Note: See the text for formulas and other assumptions underlying the calculations. The figures assume that the assignment scores are normally distributed.