Table 7.2. Required School Sample Sizes to Detect Target Effect Sizes, for Various RD Designs
Assumes a Two-Tailed Test, a Value of .15 for the Intraclass Correlations, the Cutoff Score Is at a Tertile of the Normal Score Distribution, and a 2:1 Split 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: |
|||
.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 | 142 | 91 | 52 | |
R2 = .2 | 114 | 73 | 42 | |
R2 = .5 | 71 | 45 | 26 | |
R2 = .7 | 43 | 27 | 16 | |
III: Schools: Within Districts: No Random Classroom Effects | ||||
R2 = 0 | 359 | 230 | 132 | |
R2 = .2 | 288 | 184 | 106 | |
R2 = .5 | 180 | 115 | 66 | |
R2 = .7 | 108 | 69 | 40 | |
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 | 166 | 106 | 61 | |
R2 = .2 | 133 | 85 | 49 | |
R2 = .5 | 83 | 53 | 30 | |
R2 = .7 | 50 | 32 | 18 | |
VI: Schools Within Districts: Random Classroom Effects | ||||
R2 = 0 | 462 | 296 | 170 | |
R2 = .2 | 370 | 237 | 136 | |
R2 = .5 | 231 | 148 | 85 | |
R2 = .7 | 139 | 89 | 51 | |
Note: See the text for formulas and other assumptions underlying the calculations. The figures assume that the assignment scores are normally distributed. |