P Value In Research Papers

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P Value In Research Papers



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What does the p-value mean in a research article?

The term significance level alpha is used to refer to a pre-chosen probability and the term "P value" is used to indicate a probability that you calculate after a given study. The alternative hypothesis H 1 is the opposite of the null hypothesis; in plain language terms this is usually the hypothesis you set out to investigate. For example, question is "is there a significant not due to chance difference in blood pressures between groups A and B if we give group A the test drug and group B a sugar pill?

If your P value is less than the chosen significance level then you reject the null hypothesis i. It does NOT imply a "meaningful" or "important" difference; that is for you to decide when considering the real-world relevance of your result. The choice of significance level at which you reject H 0 is arbitrary. These numbers can give a false sense of security. In the ideal world, we would be able to define a "perfectly" random sample, the most appropriate test and one definitive conclusion. We simply cannot. What we can do is try to optimise all stages of our research to minimise sources of uncertainty. When presenting P values some groups find it helpful to use the asterisk rating system as well as quoting the P value:.

The asterisk system avoids the woolly term "significant". Please note, however, that many statisticians do not like the asterisk rating system when it is used without showing P values. As a rule of thumb, if you can quote an exact P value then do. You might also want to refer to a quoted exact P value as an asterisk in text narrative or tables of contrasts elsewhere in a report. At this point, a word about error. In this article the interpretation of the p -value and some alternatives options are discussed.

In statistical science, the p -value is the probability of obtaining a result at least as extreme as the one that was actually observed in the biological or clinical experiment or epidemiological study, given that the null hypothesis is true [ 4 ]. At this point we have to introduce some terms regarding hypothesis testing. There are two hypotheses, the null and the alternative. Usually, the null hypothesis that indicates no association between the investigated factors or characteristics measured using random variables , e. On the other hand, the alternative hypothesis indicates an association between the investigated variables i. In the s, Fisher [ 5 ] proposed significance tests as a means of examining the discrepancy between the data and the null hypothesis.

In statistical theory, the p -value is a random variable defined over the sample space i. For example, a phase III clinical trial experiment is performed to determine if total cholesterol levels differ between the group that was under drug A treatment, compared with the group that was under drug B treatment. If patients were allocated to each treatment arm, and taking into account the assumptions of the appropriate significance test, the p -value of this hypothesis testing is equal to 0. The p -value of 0. Moreover, Fisher suggested 0. However, there has been considerable criticism about this choice, and its usefulness. Despite the criticisms made, all agree that the significance level should be decided before the data are viewed, and is compared against the p -value after the test has been performed.

Moreover, although p -values are widely used, there are several misunderstandings. In the text below, an attempt is made to clarify what the p -value really is and what it is not. The p -value is not the probability that the null hypothesis is true, and this is because hypotheses do not have probabilities in classical statistics. Moreover, the p -value is not the probability of falsely rejecting the null hypothesis. Falsely rejecting the mull hypothesis is a Type I error.

The p -value does not indicate the size or importance of the observed effect. Thus, a very small p -value, let say 0. Moreover, the p -value is influenced by sample size. For example, the Fig. It can be seen that if the initial sample size is doubled i. Theoretical example of p -values in relation to sample size for the same difference in the data.

Another major issue that influences medical decision making is the multiple comparisons problem which occurs when a family of statistical inferences is considered simultaneously. However, with tests performed with all null hypotheses being true, it is more likely that at least one null hypothesis will be rejected. These errors are called false positives, and many mathematical techniques have been developed to control them. For all the aforementioned reasons, many Journals have long been recommended to the authors to present confidence intervals instead of p -values since they are not considered mathematically sound [ 7 ].

Finally, the p -value is not the probability that the experiment would not yield the same conclusion after replications. For this reason Killeen [ 8 ] proposed p rep as a statistical alternative to the p -value, which calculates the probability of replicating an effect. An approximate of the p rep is the following:. The lower the p -value is, the higher the p rep. The Association for Psychological Science APS recommends to contributing authors of journals to present p rep instead of p -values. However, considerable criticism has been made. For example, p rep does not take prior probabilities into account [ 9 ], and does not bring any additional information on the significance of the result of a given experiment.

In general, Bayesian inference is a method for determining how scientific belief should be modified by observed data. Most important, Bayes factors require the addition of background knowledge to be transformed into inferences. The simplest form of Bayes factor is the likelihood ratio i. The minimum Bayes factor is objective and can be used instead of p -value as a measure of the evidential strength. However, medical researchers have not been so enthusiastic to understand and adopt that Bayesian statistical methodologies perceive a subjective approach to evidence-based analysis.

Despite the criticism, for many scientists the use of Bays factor B is an alternative to the classical hypothesis testing mentioned above. In brief, unlike p- values, Bayes factors have a sound interpretation that allows their use in both inference and decision making, since they make the distinction clear between experimental evidence and inferential conclusions while providing a framework in which to combine prior with current evidence. In this article an attempt was made to interpret the meaning of p -value, a probability that is the basis, in most biomedical research, of decision making. Recent guidelines for presenting the results of clinical experiments or observational studies, suggest providing confidence intervals instead or together with the p -values, and giving the effect sizes of the investigated associations.

Nevertheless, the p -value still has significant value when correctly interpreted and used. National Center for Biotechnology Information , U. Open Cardiovasc Med J. Published online Nov Author information Article notes Copyright and License information Disclaimer. Panagiotakos; Licensee Bentham Open.

Elie wiesel essay contest winners need to consider many contextual factors to derive scientific inferences. And when you toss it a fifth P value in research papers, and Exploring revolution essays on latin american insurgency and revolutionary theory falls Essay questions about slaughterhouse five yet again, you develop a strong suspicion that there is something wrong with the coin. References P value in research papers. American Psychological Association.