Introduction

In cohort studies, the analysis of data usually involves estimation of rates of disease in the cohort during a defined period of observation. For example, the rate of newly diagnosed cases (incidence) of cancer in a cohort per 100,000 person years of observation time. The denominator of such a rate is measured in years of observation time per person (i.e. person years).

Traditionally, cohort refers to the part of the population born during a particular period and identified by period of birth so that its characteristics such as causes of death and those still alive can be ascertained as it enters successive time and age periods. The term cohort has since been broadened to describe any designated group of persons that are followed or traced over a period of time, as in cohort study (prospective study). Thus, cohort study is synonymous with follow-up , longitudinal , or prospective study .

1. Exact Method

Because administrative data typically contain full information for birthdate, date of event, and date of death, we can use exact dates to calculate person years. However, if the exact dates of birth, entry into and/or exit from the study are unknown, these must be approximated. In this example, the person would contribute 0.5 years

Example 1


Defining a cohort of people present in Manitoba on Jan 1, 1987 and followed up to Dec 31, 1999 (or until death or loss to follow-up ). Here are two different people in the cohort.

Person 1 is present for the entire study (i.e. from Jan 1, 1987 to Dec 31, 1999).








Total person years at risk = 37.501 - 24.504 = 12.997 years.



Person 2 is present on Jan 1, 1987 but follow-up ends on Jun 20, 1992 when the event of interest takes place.








Total person years at risk = 5.467

Note: the study start date can be constant (i.e.: Jan 1, 1987) or variable for each person (i.e.: date of diagnosis of diabetes). The study end date is usually variable for each person. It is usually defined as the earliest date of:


1. Date of death
   
2. Date of event (if death is not the event of interest)
   
3. Date of loss to follow-up
   
4. Date of end of study

How much does each person contribute to 5-year age groups?



Calculation of Person Years by Age Group

Person	Age Group	Age at Start	Age at End	Person Years
1	20-<25	24.504	25	0.496
1	25-<30	25	30	5.00
1	30-<35	30	35	5.00
1	35-<40	35	37.501	2.501
2	65-<70	66.503	70	3.497
2	70-<75	70	71.970	1.970

In the example, both people contribute person years to several age groups depending on their ages at the beginning and end of the study. All the intermediate age groups between age at start and age at end must be passed through in order to calculate the person years accurately. For each age group, age at the start and age at the end must be calculated.

2. Approximate Method

If the exact dates of birth, entry into and/or exit from the study are unknown, the missing information must be approximated. For example, if the only information we have for a person is their age at entry, and years of entry and exit, calculate the years s/he contributed according to the following:


• 0.5 year for entry year
  
• 0.5 year for exit year
  
• 1 year for each year between entry & exit years

Note : If a person enters and leaves a study during the same year, s/he has contributed .25 year.