Introduction to incidence rates and time-changing variables for cohort studies

In orthodontics and dentistry in general, we are frequently working on studies where groups of patients are followed over time with the aim to record the occurrence of an event. Subjects may enter a study at various times and be followed until (1) the event of interest occurs, (2) the study is completed (end of follow-up time), or (3) they are lost to follow-up. The analysis of this kind of study is based on incidence rates, or more simply, rates (λ). Rate is the total number of events per person per unit of time (ie, person-time) and reflects the probability that a subject will experience the event of interest at a prespecified time.

Rate λ=total number of(new)eventstotal persontime at risk
Rate λ = total number of ( new ) events total person – time at risk

The unit of rate is the number of events per person-time; depending on what is more sensible for interpretation purposes, time can be measured in months, years, and so on, and persons can be counted per 100 or 1000. For example, if the number of implant failures is on average 20 in a population of 100 persons over a 2-year period, then

Rate λ=20100*2=0.1events per personyears=10events per100personyears
Rate λ = 20 100 * 2 = 0.1 events per person – years = 10 events per 100 person – years
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Dec 19, 2018 | Posted by in Orthodontics | Comments Off on Introduction to incidence rates and time-changing variables for cohort studies
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