## Sunday, August 25, 2013

### Concept of "Person Year" in daily life

A while ago, I wrote a blog “Understanding person-year or patient-year”. While the concept of ‘person-year’ or ‘patient-year’ is mainly used in the epidemiology research (especially in the occupational health field) and clinical trials, the same concept may be used in the daily life.

This morning, I read an “Ask Marilyn” column on Parade.com. The question and answer applies the same concept of the ‘person-year’ or ‘patient-year’. With the concept of “person-year”, we need to combine the number of persons and the number of follow-up years in order to do the further calculation/statistical analyses. In the question/answer below, the number of family members and the number of days each family member stays need to be combined before the further calculation.

While 'person-year' may be more frequently used in the research field, the unit of time can be in years, months, days and they also can be called 'Person-time'. Person-time is an estimate of the actual time-at- risk in years, months, or days.
Question:
Nine family members will be renting a vacation property. The fee is \$3,600 for 10 days. People will be staying for a varying number of days. I say the first step in figuring what each person owes is to divide the fee by nine. My husband says we should start by dividing the fee by 10—the number of days we have the rental. Who is right?

Marilyn responds:
Neither of you, but the dilemma is common. Here’s a way for anyone to solve this kind of vacation problem with any number of guests, days, etc. I’ll use your case as an example, and I’ll assume a family member who’s there surrounded by loved ones pays the same per day as one who gets the place all to him- or herself.
First, add up the number of days each family member stays. (Let’s say your nine people stay a total of 5+6+6+8+8+9+10+10+10 = 72 days.) Then divide the total rental fee by that figure (\$3,600 ÷ 72 days = \$50 per “person-day”). Each family member owes that result (\$50) multiplied by the number of days he or she stays. So in this example, a person who stays five days owes \$250. A person who stays all 10 owes \$500.