Use the two examples on this page to show that using block information allows you to estimate treatment differences more accurately.
In both examples, the initial analysis ignores the blocks, so the 95% CIs for differences between the control and other treatments are wide.
Click Show blocks of subjects then select Correct
analysis using paired differences from the pop-up menu. The top diagram now shows paired differences between the treatments and control within each block, and the 95% CIs for the differences (based on the differences) are much narrower, giving much clearer evidence of differences between the treatments.
Codeine and acupuncture for dental pain relief (randomised
blocks)
- Experimental units
- 32 subjects, grouped into blocks of 4 by an initial assessment of their
tolerance to pain.
- Treatments
- Four treatment combinations of (codeine or a sugar capsule) and (active
or inactive acupuncture points) randomised in each block of 4. Double-blind.
- Response
- Pain relief scores two hours after dental treatment.
|
Pain relief score |
Tolerance
group |
Control |
Codeine
only |
Acupuncture
only |
Codeine +
Acupuncture |
|
0.0
0.3
0.4
0.4
0.6
0.9
1.0
1.2 |
|
0.6
0.7
0.8
0.9
1.5
1.6
1.7
1.6 |
|
0.5
0.6
0.8
0.7
1.0
1.4
1.8
1.7 |
|
1.2
1.3
1.6
1.5
1.9
2.3
2.1
2.4 |
|
Effect of nicotine on tics (repeated measures)
- Experimental units
- 10 patients with Tourette's syndrome, a neurological disorder that results
in tics.
- Measurements
- Number of tics during an initial 30-minute period (baseline), during 30
minutes of chewing nicotine gum, from 0 to 30 minutes after chewing, and finally
from 30 to 60 minutes after chewing.
|
Number of tics during 30-min period |
Patient |
Baseline |
Chewing gum |
0-30 min after |
30-60 min after |
|
249
1095
83
569
368
326
324
95
413
332 |
|
108
593
27
363
141
134
126
41
365
293 |
|
93
600
32
342
167
144
312
63
282
525 |
|
59
861
61
312
180
158
260
71
321
455 |
|