Có 3 thiết kế nghiên cứu: a. Thực nghiệm; b. Thuần tập hồi cứu; c. Ngang; "Giá trị suy luận căn nguyên" tùy vào thiết kế nghiên cứu sẽ giảm dần theo trình tự:

Nghiên cứu nửa dọc (hoặc nghiên cứu theo nhóm tuổi - cohort study) theo dõi một nhóm người (cohort) trong một khoảng thời gian. Điểm đặc biệt của nghiên cứu nửa dọc là các đối tượng trong cùng một cohorte được khảo sát nhiều lần để theo dõi sự thay đổi theo thời gian. Do đó, đáp án đúng là "Nhiều lần".

Nghiên cứu thuần tập (cohort study) theo dõi một nhóm người theo thời gian để xem xét sự phát triển của bệnh hoặc kết quả quan tâm. Vì dữ liệu về phơi nhiễm được thu thập trước khi bệnh phát triển (tiến cứu), sai số nhớ lại (recall bias) ít có khả năng xảy ra hơn so với các nghiên cứu quan sát khác, đặc biệt là nghiên cứu bệnh chứng, nơi thông tin về phơi nhiễm được thu thập sau khi bệnh đã xảy ra và người bệnh có thể nhớ lại khác với nhóm chứng. Do đó, sai số nhớ lại trong nghiên cứu thuần tập thường thấp hơn so với các nghiên cứu quan sát khác.

Nghiên cứu bệnh chứng là một loại nghiên cứu dịch tễ học quan sát, trong đó các đối tượng được chọn dựa trên tình trạng bệnh (nhóm bệnh) hoặc không bệnh (nhóm chứng). Sau đó, người ta thu thập thông tin về các yếu tố nguy cơ hoặc phơi nhiễm trong quá khứ để xác định xem có mối liên hệ nào giữa các yếu tố này và bệnh hay không. Do đó, nó là một loại nghiên cứu hồi cứu, vì nó nhìn lại quá khứ để tìm kiếm các yếu tố có thể liên quan đến bệnh hiện tại.

Nghiên cứu bệnh chứng dễ bị sai số chọn (selection bias) hơn so với các nghiên cứu quan sát khác như nghiên cứu thuần кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого кого 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Nghiên cứu bệnh chứng dựa vào việc thu thập thông tin về tiền sử phơi nhiễm của những người mắc bệnh (nhóm bệnh) và những người không mắc bệnh (nhóm chứng). Vì vậy, những người tham gia nghiên cứu (đặc biệt là nhóm bệnh) thường phải nhớ lại những sự kiện, hành vi hoặc phơi nhiễm đã xảy ra trong quá khứ. Sai số nhớ lại (recall bias) xảy ra khi có sự khác biệt có hệ thống trong độ chính xác hoặc đầy đủ của thông tin được nhớ lại giữa các nhóm nghiên cứu. Trong nghiên cứu bệnh chứng, sai số này thường cao hơn so với các nghiên cứu quan sát khác do nhóm bệnh có xu hướng nhớ lại các phơi nhiễm có thể liên quan đến bệnh tật của họ nhiều hơn so với nhóm chứng.