nghien cuu benh chung:

- lua chon 2 nhom ca the , 1 nhom ca the mac benh, 1 nhom ca the khong mac benh. tien hanh dieu tra hoi cuu de xac dinh lai qua trinh phoi nhiem yeu to nguy co.

- uu diem: tuong doi de, ton it thoi gian, cho phep phan tich nhieu yeu to.

- nhuoc diem: + de co sai so nho lai, +) kho xay dung mot nhom chung hoan chinh. +) khong du tai lieu ho so, ho so khong hoan chinh. +) can phai co tieu chuan chuan doan benh chinh xac.

Nghien cuu thuan tap:

- lua chon vao thoi diem nghien cuu 1 nhom ca the phoi nhiem voi yeu to X va mot nhom ca the khong phoi nhiem voi yeu to X. theo doi o trong tuong lai de xem benh nhan co mac benh B hay khong.

- Nhuoc diem: ton kem, co nhieu nguoi bo cuoc, nguy hiem.

- uu diem: kha chinh xac, rat co y nghia trong phat hien moi lien quan giua yeu to nguy co va benh.