2. There is a large area of Bangladesh where the groundwater is contaminated by

Question

2. There is a large area of Bangladesh where the groundwater is contaminated by arsenic. Residents of this area suffer from elevated rates of cancer, and of pre- cancerous skin lesions, because they often have no alternative but to drink the arsenic-contaminated water. Many efforts are underway to solve this problem. In one study, investigators looked for evidence that the effect of arsenic on pre- cancerous skin lesions might be increased or decreased by other factors efect modifiers. One such possible factor is pesticide use which is common in this agricultural region. The table shows the results of a 6-year closed prospective cohort study in which residents were followed and periodically examined for the occurrence of pre-cancerous skin lesions. All these people are exposed to arsenic in drinking water, and so this table does not show that exposure. These data divide the cohort into 3 groups based on how long they were exposed to 9 pesticides Pesticide use (years) Total Followed Skin Lesions No Lesions 0 less than 7 7 and over 1924 939 1114 138 250 1702 801 864 What measure of disease frequency can be calculated in these data?

Explanation / Answer

A) We can calculate the Incidence rate/Proportion of the occurrence of lesion using this data.

Incidence rate : It is the fraction of individuals exposed out of the total observed individuals. It can be expressed as a percentage as well.

B) Not Exposed:

Incidence rate = #Skin Lesions/Total followed

= 222/1924 = 0.115 or 11.5%

Exposed(0-7):

Incidence rate = #Skin Lesions/Total followed

= 138/939 = 0.147 or 14.7%

Exposed(7+):

Incidence rate = #Skin Lesions/Total followed

= 250/1114 = 0.224 or 22.4%

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