Table 1.

Eight scenarios depicting cross-sectional studies carried out in different underlying conditions, yet all uncovering prevalence ratios between exposed and non-exposed groups of 2.0.

Scenario	Type of outcome event	Duration of outcome (${\overline{T}}_i$)	IDR *		CIR **
1	Rare	Long and equal	2.053	(+2.6)	2.025	(+1.2)
2	Rare	Long and unequal	0.205	(-89.7)	0.227	(-88.6)
3	Rare	Short and equal	2.053	(+2.6)	1.809	(-9.5)
4	Rare	Short and unequal	0.205	(-89.7)	0.443	(-77.8)
5	Common	Long and equal	3.000	(+50.0)	2.230	(+11.5)
6	Common	Long and unequal	0.300	(-85.0)	0.656	(-67.2)
7	Common	Short and equal	3.000	(+50.0)	1.037	(-48.1)
8	Common	Short and unequal	0.300	(-85.0)	1.000	(-50.0)

* In brackets: % bias (IDR) calculated as [-(PR - IDR )/PR]*100, where PR = 2.0 is fixed.

** In brackets: % bias (CIR) calculated as [-(PR - CIR )/PR]*100, where PR = 2.0 is fixed.

Time units used in the scenarios

Average duration of outcome long and equal → T₁ = T₀ = 1

Average duration of outcome long and unequal → T₁ = 1; T₀ = 0.1

Average duration of outcome short and equal → T₁ = T₀ = 0.1

Average duration of outcome short and unequal → T₁ = 0.1; T₀ = 0.01

Time interval for projecting CIR → Δt = 1

Prevalences according to exposure group and outcome frequency

Rare outcome → P₁ = 0.05; P₀ = 0.025

Common outcome → P₁ = 0.5; P₀ = 0.25