Table 7.1. Required School Sample Sizes to Detect Target Effect Sizes, for Various RD Designs

List of Tables

Assumes a Two-Tailed Test, a Value of .15 for the Intraclass Correlations, the Cutoff Score Is at the Center of the Normal Score Distribution 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	39	25	14
R² = .2	31	20	12
R² = .5	20	13	7
R² = .7	12	8	4
II: Classrooms Within Schools: Fixed School Effects
R² = 0	141	90	52
R² = .2	113	72	41
R² = .5	71	45	26
R² = .7	42	27	16
III: Schools: Within Districts: No Random Classroom Effects
R² = 0	357	229	131
R² = .2	286	183	105
R² = .5	179	114	66
R² = .7	107	69	39
IV: Students Within Schools: Random Site Effects
R² = 0	63	40	23
R² = .2	50	32	18
R² = .5	31	20	12
R² = .7	19	12	7
V: Classrooms Within Schools: Random School Effects
R² = 0	165	105	61
R² = .2	132	84	48
R² = .5	82	53	30
R² = .7	49	32	18
VI: Schools Within Districts: Random Classroom Effects
R² = 0	459	294	169
R² = .2	367	235	135
R² = .5	230	147	84
R² = .7	138	88	51
Note: See the text for formulas and other assumptions underlying the calculations. The figures assume that the assignment scores are normally distributed.