Water and Long Haul Statistics

Last week the paper Water intake and post-exercise cognitive performance: an observational study of long-distance walkers and runners was was published and I am one of the authors as Margo felt the analysis was going to be tricky. I suppose I could have talked about it then but last week was dominated by the move, so here is my reflection as the statistician on the project.

It was a paper that started out as a student project, well two student projects actually although in the end  it was only water we concentrated on and the other half which looked at nutrition in general did not. What was unusual about this study was that it was a field trial (to adopt the agricultural terminology) not a laboratory/clinic based trial. Field trials are known to have large levels of complexity and this was the case here.

Lets start with the basics, this trial was pre and post race measures of cognitive performance for long distance runners. The students had no ability to determine who went into which race, that was the choice of the athlete who decided to join the study after they had entered a race where the students were attending. Secondly we had no control over liquid intake prior or during the race we had to rely on self reporting by the athletes. Thirdly we found there are few reliable ways of finding someone's hydration status in a one off setting. Most of the highly technical methods either are not appropriate for a race situation or are only reliable if repeatedly administered.

This is one of those case where it looks simple, a paired T-test will do, and it quickly comes apparent that no it won't do. The biggest problem was removing the confounding. I went through several analyses where the results were incoherent (did not make sense), yet consistent enough to suggest there was something going on but not enough so I could satisfactorily characterize it. So I had to resit down and consider the assumptions we had made and then re-analyze. I think it was on fifth attempt that it finally hit me that we had a problem with pre-scores and the water intake pre-race. If cognitive scores are dependent on hydration then this is true of pre-race ones as well as post-race. So to get a cognitive performance score that was neutral before the race I had to adjust the scores for the water intake over the previous twenty four hours. I could not just use the raw scores. It worked, at last we got a consistent and coherent story. Then all we had to do was to write the paper.

However in writing it we discovered that we should have taken pre and post weight, I suspect as well as self reported normal weight. It is one of those cases where

It took one year from student placement to analysis and another year for the paper to be produced, but what had been very much a first simple trial at doing something had born fruit.