From: Experience collecting interim data on mortality: an example from the RALES study
 |  |  | Observed |  |  |  |  | ||
---|---|---|---|---|---|---|---|---|---|
 | Data | Meeting |  |  |  | Estimated | z-value | Observed | |
Look | cutoff | date | Placebo | Treatment | Relative | information | P | Â | Â |
no. | (d-mo-y) | (d-mo-y) | deaths | deaths | risk | time* | Critical | Observed | (two-sided) |
Interim analysis cut-offs with the sweeps as they occurred | |||||||||
1 | 09-8-96 | 24-8-96 | 70 | 52 | 0.76 | 0.12 | 6.38 | 1.58 | 0.11 |
2 | 10-3-97 | 17-3-97 | 136 | 109 | 0.83 | 0.24 | 4.43 | 1.69 | 0.092 |
3 | 14-8-97 | 25-8-97 | 224 | 175 | 0.80 | 0.34 | 3.67 | 2.55 | 0.011 |
4 | 26-3-98 | 30-3-98 | 304 | 241 | 0.81 | 0.48 | 3.04 | 3.02 | 0.0026 |
5 | 14-8-98 | 24-8-98 | 351 | 269 | 0.78 | 0.57 | 2.79 | 3.75 | 0.00018 |
Estimated interim analysis cutoffs without the sweeps†| |||||||||
4a | 26-3-98 | 30-3-98 | 281 | 222 | 0.81 | 0.45 | 3.16 | 2.93 | 0.0034 |
 |  |  | (275-289) | (214-228) | (0.76-0.85) |  |  | (2.33-3.69) |  |
5a | 14-8-98 | 24-8-98 | 333 | 256 | 0.79 | 0.55 | 2.81 | 3.59 | 0.00034 |
 |  |  | (325-341) | (248-264) | (0.74-0.83) |  |  | (2.87-4.33) |  |
Interim analysis cutoffs that would have occurred if we had known the true numbers and times of deaths | |||||||||
1b | 09-8-96 | 24-8-96 | 81 | 59 | 0.75 | 0.14 | 5.88 | 1.80 | 0.072 |
2b | 10-3-97 | 17-3-97 | 189 | 140 | 0.76 | 0.26 | 4.24 | 2.75 | 0.0060 |
3b | 14-8-97 | 25-8-97 | 257 | 201 | 0.80 | 0.38 | 3.46 | 2.82 | 0.0048 |
4b | 26-3-98 | 30-3-98 | 330 | 254 | 0.79 | 0.51 | 2.95 | 3.56 | 0.00038 |
5b | 14-8-98 | 24-8-98 | 383 | 283 | 0.76 | 0.60 | 2.72 | 4.46 | 0.000008 |