In a recent JAMA Oncology Patient Page , oncologist H. Jack West and biostatistician Suzanne Dahlberg explain several aspects of cancer clinical trials. I was struck in particular by the description they crafted of p-values:
Statistics Let Us Describe Differences in Outcomes Between Groups
Results for different end points are compared in a few ways to assess whether one strategy is better than another. Differences are considered statistically significant if the probability of the differences seen occurring by chance, based on mathematical calculations, is below a threshold usually defined as 5%: this is also reflected as a P value of .05 or lower (1.00 representing 100% probability of chance explaining the findings).
If one must describe P values in other than scourging terms, this seems to me about as good a job as one could do. Have I overlooked something? It does not escape me how delicate this description is. I note for example how it would collapse with the subtlest change of verb tense—from “occurring” to “having occurred”. Also, I appreciate there’s an awful lot packed into the deus ex machina of “based on mathematical calculations.” But still … wasn’t this as good a job as can be done with this thankless task?
How would you describe P values to a cancer patient advocate who is (as many do) laboring to read the trial literature and make sense of it?
- West H, Dahlberg S. Clinical Trials, End Points, and Statistics—Measuring and Comparing Cancer Treatments in Practice. JAMA Oncol . September 2018. doi:10.1001/jamaoncol.2018.3708