Schistosomiasis is a parasitic infection that is spread when larval forms of the parasite penetrate the skin during contact with infested water. You are working in a village of 205 people, where it is hypothesized that spread of schistosomiasis is occurring through contact with local well water. You find that 125 of the villagers use the suspected well, while the rest of the villagers do not use the well. After gathering urine samples, you find that 104 of the villagers who use the well are infected with schistosomiasis. Only 15 of the villagers who do not use the well are infected. I: Calculate the relative risk. II: Interpret your results. III. Calculate the attributable risk. IV. Interpret your results.

1. Relative risk: proportion of the disease after exposition / proportion of the disease without exposition

= (104/125) / (15/125)

= 83.2 / 12  = 6.93  

The relative risk is calculated as the probability of having the disease in one exposed group when it is compared to the expected outcome in the non-exposed group. A relative risk of 6.93 indicates a risk of almost seven times higher than the average risk, being its value very high

2. Attributable Risk (AR) for an exposed group:  

the incidence in the exposed group - incidence in the non exposed group/incidence in the non exposed group

AR = 104 -15 /104

AR= 0.85

The relative risk is useful to determine whether the local well water is associated with the parasitic infection, while attributable risk allows to determine how much of schistosomiasis infection may be attributable to the exposition with the local well water. In this case, a AR of 0.85 is considered to be high and therefore people's contact with suspected well water should be avoided