P Values (Calculated Probability) and Hypothesis Testing ... | p value 1
ThePvalue,orcalculatedprobability,istheprobabilityoffindingtheobserved,ormoreextreme,resultswhenthenullhypothesis(H0)ofastudyquestionistrue–thedefinitionof‘extreme’dependsonhowthehypothesisisbeingtested.PisalsodescribedintermsofrejectingH0whenitisactuallytrue,however,itisnotadirectprobabilityofthisstate. Thenullhypothesisisusuallyanhypothesisof"nodifference"e.g.nodifferencebetweenbloodpressuresingroupAandgroupB.Defineanullhypothesisforeachstudyquestionclearlybeforethestartofyourst...
The P value, or calculated probability, is the probability of finding the observed, or more extreme, results when the null hypothesis (H0) of a study question is true – the definition of ‘extreme’ depends on how the hypothesis is being tested. P is also described in terms of rejecting H0 when it is actually true, however, it is not a direct probability of this state.
The null hypothesis is usually an hypothesis of "no difference" e.g. no difference between blood pressures in group A and group B. Define a null hypothesis for each study question clearly before the start of your study.
The only situation in which you should use a one sided P value is when a large change in an unexpected direction would have absolutely no relevance to your study. This situation is unusual; if you are in any doubt then use a two sided P value.