Table 2. Validity Assumptions for Use of Case-Crossover and Self-Controlled Case Series Designs.

Case-crossover studies	N = 50
Assumptions	Studies No.(%)
Acute outcome	43 (86)
Rare event	32 (64)
Intermittent exposure	36 (72)
The opportunity of event is the same during case and control time-period(s)	47 (94)
No time-trend in exposure or CTC use	34 (68)
All assumptions fulfilled	12 (24)
Self-controlled case series	N = 45
Assumptions	Studies No. (%)
Rare or recurrent event	45 (100)
Only one event per subject or independence between two consecutive events was defined	39 (87)
Intermittent exposures	35 (78)
Probability of further exposure not affected by previous events	24 (53)
Event did not affect the short term mortality probability	39 (87)
Collections of events and exposures was independent	42 (93)
All assumptions fulfilled	18 (40)