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A Methodology for the Health Sciences - part 3

Khi kết hợp các bảng không có hiệp hội, hoặc tỷ lệ tỷ lệ cược của 1, sự kết hợp có thể hiển thị liên kết. Ví dụ, người ta sẽ mong đợi để tìm một mối quan hệ tích cực giữa bệnh ung thư vú và là một bà nội trợ. | COMPARING TWO PROPORTIONS 171 and for stratum 2 Exposure Disease - 40 60 40 60 _ 40 60 2 1 2 40 60 In both tables the odds ratio is 1 and there is no association. Combining tables the combined table and its odds ratio are Exposure Disease - 45 110 50 160 combined 45 160 1 . 50 110 When combining tables with no association or odds ratios of 1 the combination may show association. For example one would expect to find a positive relationship between breast cancer and being a homemaker. Possibly tables given separately for each gender would not show such an association. If the inference to be derived were that homemaking might be related causally to breast cancer it is clear that one would need to adjust for gender. On the other hand there can be an association within each stratum that disappears in the pooled data set. The following numbers illustrate this Stratum 1 Disease 60 50 3 10 100 Exposure - 1 60 100 10 50 Stratum 2 Disease 50 60 3 100 10 Exposure - 2 50 10 100 60 Combined data Disease Exposure - combined 1 110 110 110 110 172 COUNTING DATA Thus ignoring a confounding variable may hide an association that exists within each stratum but is not observed in the combined data. Formally our two situations are the same if we identify the stratum with differing groups. Also note that there may be more than one confounding variable that each strata of the third variable could correspond to a different combination of several other variables. Questions of Interest in Multiple 2 X 2 Tables In examining more than one 2 X 2 table one or more of three questions is usually asked. This is illustrated by using the data of the study involving cases of acute herniated lumbar disk and controls not matched in Example 6.15 which compares the proportions with jobs driving motor vehicles. Seven different hospital services are involved although only one of them was presented in Example 6.15. Numbering the sources from 1 to 7 and giving the data as 2 X 2 tables the tables and the seven