Tuesday, April 5, 2016

The value of the P-value

Scientist tend to place a lot of importance in the P value. PIs loose grants over P values, grad students have nervous break downs over “insignificant data”, undergraduates desperately look for outliers they can redo to correct that p-value. Is the P-value really that important?

As we have learned in class this semester, the p-value is the probability of obtaining a result equal to or more extreme than predicted with the null hypothesis. So a low p-value indicates that it was unlikely that random chance gave you such an outlying result. Many scientist would stop when they see that low number, wipe the sweat off their brows, and submit their grant or paper.

However is a p-value of 0.051 really all that much worse than a p value of 0.049? Is the difference between those p-values really worth a nervous breakdown? The p-value is an arbitrary line drawn by scientists, and often young scientists fail to look beyond it. A p-value is just one statistical number that can be used in conjunction with other statistical values such as the mean, confidence interval, and error, to make a decision about the implications of an experiment.

A scientist needs to use his/her judgment to conclude if an experiment is worth pursuing further or if it is a waste of time; a significant or insignificant p value should not make that decision. An insignificant p-value does not mean the data was worthless, but only a clear minded scientist can determine what it actually means. Likewise, but much less considered, a significant p-value does not mean that the experiment showed a result worth celebrating over. Your significant p-value might indicate that your drug decreased the stress levels of mice 10%, but is a 10% decrease in mice stress really significant for any application?

These sorts of questions are what scientists should be thinking about. In one experiment, a p-value may hold a lot of weight, but in another experiment, the confidence interval might tell much more about the results. Scientist must be able to analyze their data, not just statistically, but critically to know what their statistics are actually telling them.

1 comment:

  1. Great post! I completely agree. A scientist's most unique skill is to think critically about the problems and hypotheses they are interested in (as well as thinking EXTRA critically about other people's problem's and hypotheses). However, this ability seems to completely disappear at the end of an experiment, when most of us find ourselves staring at the computer and hoping at least one asterisk pops up in our analysis. I wonder if this is a cognitive issue, a cultural issue, or both. Regardless, I agree that we as a community need to be more conscious about fostering important research rather than just "significant" research.