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    Next example

    subjects:
    smokers: 1000
    non smokers: 1000
    of same age range, sex ratio

    status of lung cancer for 5 years were observed.

Next example is study about relationship between smokers and lung cancer.

If you like to examine if exposure to something is really related with onset of disease, you have to collect people with same back ground other than exposure.

suppose that we could collect 1000 person in each smokers and non smokers group and the results are obtained in 5 years.