Có 3 thiết kế nghiên cứu: a. Ngang; b. Tương quan; c. Trường hợp; "Giá trị suy luận căn nguyên" tùy vào thiết kế nghiên cứu sẽ tăng dần theo trình tự:

Thiết kế nghiên cứu thực nghiệm (a) có giá trị suy luận căn nguyên cao nhất vì có sự can thiệp và kiểm soát các yếu tố gây nhiễu. Thiết kế thuần tập hồi cứu (b) có giá trị suy luận căn nguyên thấp hơn do dựa vào dữ liệu quá khứ và khó kiểm soát các yếu tố gây nhiễu. Thiết kế ngang (c) có giá trị suy luận căn nguyên thấp nhất vì chỉ thu thập dữ liệu tại một thời điểm, khó xác định mối quan hệ nhân quả. Do đó, trình tự giảm dần về giá trị suy luận căn nguyên là a, b, c.

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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