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Patients will be measured 1. during an operation and 2. after the operation.

Measurement 1: During the operation with machine1 to assess whether the transplanted tissue is at the right location (Outcome is binary (right location vs wrong location)).

Measurement 2: Measurements will be performed 1 months after the operation with machine2 to assess whether the transplanted tissue is still at the right location and did not migrate (Outcome is binary, right/wrong location).

Research question: How well does a positive/negative result during measurement 1 predict the outcome of having the tissue at the right/wrong location at measurement2?

Which test is appropriate to answer the research question? I use SPSS.

Thank you for your time, I am looking forward to your replies. -Leif

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  • $\begingroup$ do you have only two variables? measurement 1 and measurement 2? $\endgroup$
    – Yuval Spiegler
    Commented Sep 13, 2017 at 9:28
  • $\begingroup$ @YuvalSp That is correct. But the measurements after the operation will be at multiple time points (1 week, 1 month, 3 months), however, if any of these show a change in location,the treatment will be considered 'failed', so the outcome is still binary: During operation: 1 for good location, 0 for wrong location After operation: At all time points good location: 1; at any time point wrong location: 0. I hope that makes it clear. (in regards to the edit: I wasn't aware that the return key will post the comment) $\endgroup$
    – Leif_S
    Commented Sep 15, 2017 at 7:15

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You probably want to use McNemars test, which is a type of binary contingency test for paired data (like Chi Square taking into account within subject measurements). In SPSS go to:

Analyze $\rightarrow$ Nonparametric Tests $\rightarrow$ Legacy Dialogs $\rightarrow$ 2 Related Samples

enter image description here

If the test is significant ($p<0.05$ or whatever value you use) than the locations are not randomly distributed, meaning that the right/wrong location during surgery is likely influencing the location post-surgery.

Edit: If one of the cells in the 2x2 contingency table has 0 in it, but not due to any problem with the data itself - it is not a problem. This can be due to the fact that all of the patients who had the wrong location during operation have a wrong location post-operation (0 in the wrong-right cell). This will likely translate to a significant test.

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  • $\begingroup$ Thanks for the insight! One follow-up question: Is it valid to use McNemars if one cell in the 2x2 table is empty (=0)? $\endgroup$
    – Leif_S
    Commented Sep 15, 2017 at 7:28
  • $\begingroup$ see answer in edit above. $\endgroup$
    – Yuval Spiegler
    Commented Sep 15, 2017 at 12:51
  • $\begingroup$ I ran the analysis and did not reach significane. However, I just want to be clear about the interpretation. The goal of the study is to test how well the pretest can predict the posttest result. So now that I did not reach significane, it means that my pretest does predict the posttest, since people that stayed in location1 remain at location1 in the posttest. If I did reach significane it would mean that patients switched from one group to the other (e.g. from right location to wrong location). Is my interpretation correct? $\endgroup$
    – Leif_S
    Commented Sep 24, 2017 at 17:36

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