Alternatively, differences can be calculated between the estimates for the two groups. The difference can be reported with a confidence interval that includes upper and lower bounds. If the confidence interval includes 0, this indicates that there is no significant difference between the groups. If the interval does not include 0, there is an increased risk for one population compared to the other (or conversely, a decreased risk). The difference can convey an excess or decreased risk among the exposed group due to exposure, possibly an excess or decreased risk that would be removed if the exposure ends, a potential reduction in risk for exposed individuals, or the absolute risk of the exposure.
Differences: Disease Frequency(Population A)  Disease Frequency(Population B)
Difference Calculations Section
Risk Difference
Cumulative incidence difference
 More generally can be thought of as:
 Difference of Disease Incidence= [A/(A+B)]  [C/(C+D)]
 In our pollution example, this would be [291/1351]  [232/1631] = 21.5%  14.2% = 7.3%. Thus, participants from highpollution cities have a 7.3% higher risk of death than participants from lowpollution cities.
Incidence rate difference
 In our pollution example, this would be (16.24/1000 personyears)  (10.72/1000 personyears) = 5.51/1000 personyears. Thus, there are 5.51 excess deaths per 1000 personyears among those in the high pollution city. Alternatively, the number of deaths could be reduced by 5.51 per 1000 personyears, if the pollution level in the highpollution city was reduced to that of the lowpollution city.
Attributable Proportion among the Total Population (AP_{t})
(Also known as population attributable risk (PAR))
The Attributable Proportion among the Total Population depends upon the prevalence of the exposure in the study population. This value is often used to convey implications for policy or regulations.
General Formula
 \(\mathrm{AP}_{\mathrm{t}}=\dfrac{\text { Risk(study population)Risk(unexposed group) }}{\text { Risk(study population) }}\)
Cumulative incidence AP_{t}

In our example this would be [(523/2982)  (232/1631)]/(523/2982) = 0.189. Thus 18.9% of the deaths in the population are attributable to the high pollution levels, and thus would be eliminated if the pollution levels were reduced.
Incidence rate AP_{t}
 First, we need to calculate the incidence rate in the entire population. This would be the sum of all the deaths (1430) divided by the sum of all the personyears (111076) = 12.87 deaths per 1000 personyears.
 In our example, this would be [(12.87/1000 personyears  10.73/1000 personyears)] / (12.87/1000 personyears) = 0.166. Thus 16.6% of the deaths in the population are attributable to the high pollution levels, and thus would be eliminated if the pollution levels were reduced.