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Nope, I think you may have missed a paragraph:

> but all participants were included for statistical analysis of the primary outcome (intention-to-treat approach).

I think this study looks rather solid.

I think that's a valid point but it may not make a significant difference. Judging by the fact that the control group had a pretty decent drop-out rate it appears that the drop outs reverted to lower than average physical activity levels. I am playing devil's advocate here, but you can't rule out the hypothesis that trying a bit of HIIT convinced more people to take it easy at their age.

I think I see the point you are making now. If HIIT to a greater degree prompts less fit participants to drop out than in the other groups, it might skew the results. Not really sure in what direction though. If it had been a study on the effect of HIIT vs less intense training on cardiovascular fitness, this kind of dropout would bias the results in the direction of HIIT. I have no idea of what effect it would have when the variably of interest is mortality.

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