Table 115Defining summary statistics

Summary statisticDefinition
Incidence rateIncident cases divided by the number in the cohort multiplied by the exposure time
Standardised incidence (SIR)/rate ratio (SRR)/Standardised morbidity ratio (SMR)Incidence rate observed among exposed divided by the incidence rate expected in a matched population
Relative standardised incidence/rate ratio
Relative standardised morbidity ratio
Ratio between two standardized rate ratios (takes into account the difference in excess risk vs matched general population between two subgroups)
Incidence rate ratio (IRR)Incidence rate among exposed divided by the incidence rate among non-exposed (direct comparison of risk between two subgroups)

From: 9, Phototherapy

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Psoriasis: Assessment and Management of Psoriasis.
NICE Clinical Guidelines, No. 153.
National Clinical Guideline Centre (UK).
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