Question

Epidemiology_April

To evaluate possible risk factors for breast cancer,investigators conducted a case-control study Cases: 1000 women with breast cancer who had previously given birth using a tumor registry list from Massachusetts Controls:1000 women without breast cancer who had previously given birth were selected at random from voter registration lists All subjects were interviewed on a variety of exposures including reproductive characteristics,demographic information,and past medical history 525 of those with breast cancer reported having an age at first birth of 35 years or younger 635 women without breast cancer reported having their age at first birth as 35 years or younger.

i) Construct a 2x2 table that summarizes these data on the association between age at first birth of 35 years or younger versus older than 35 years,and breast cancer.

ii) What measure of association would you use to describe the relationship between age at first birth and breast cancer?

iii) Calculate that measure of association for this study and interpret this value

Answer 1

Answer (i)

2x2 table that summarizes these data is as follows:

Answer (ii)

The measure of association that would be used to describe the relationship between age at first birth and breast cancer is Odds Ratio. This is because an odds ratio (OR) is a measure of association between an exposure and an outcome.

Answer (iii)

We will calculate odds ratios (OR) using a two-by-two frequency table

Where

• a = Number of exposed cases

• b = Number of exposed non-cases

• c = Number of unexposed cases

• d = Number of unexposed non-cases

OR= (a/c)/(b/d) = ad/bc

OR = (525*365)/(475*635) = 0.64

Interpretation:

Women who have an early age at first birth have 0.64 times the odds (or 64% of the odds, or 36% less odds) of developing breast cancer compare with those who have a late age at first birth.