Question

In your own words, explain what a P-value means and why it is important to calculate.

Answer 1

• The P value is defined as the probability under the assumption of no effect or no difference (null hypothesis), of obtaining a result equal to or more extreme than what was actually observed. The P stands for probability and measures how likely it is that any observed difference between groups is due to chance. Being a probability, P can take any value between 0 and 1. Values close to 0 indicate that the observed difference is unlikely to be due to chance, whereas a P value close to 1 suggests no difference between the groups other than due to chance. Thus, it is common in medical journals to see adjectives such as “highly significant” or “very significant” after quoting the P value depending on how close to zero the value is.
• A small p-value (typically ≤ 0.05) indicates strong evidence against the null hypothesis, so you reject the null hypothesis. A large p-value (> 0.05) indicates weak evidence against the null hypothesis, so you fail to reject the null hypothesis.
• Now we understand what p-value means with the help of an example. P-value in pizza delivery time.We’ve collected some sampled delivery times, we perform the calculation and find that the mean delivery time is longer by 10 minutes with a p-value of 0.03.What this means is that in a world where the pizza delivery time is 30 minutes or less (null hypothesis is true), there’s a 3% chance we would see the mean delivery time is at least 10 minutes longer due to random noise.The lower the p-value, the more meaningful the result because it is less likely to be caused by noise. After analysing the sample delivery times collected, the p-value of 0.03 is lower than the significance level of 0.05 (assume that we set this before our experiment), and we can say that the result is statistically significant. Because we’ve always been believing the pizza place that it can fulfil its promise to deliver pizza in 30 minutes or less, we now need to think if this belief still makes sense since the result tells us that the pizza place fails to deliver its promise and the result is statistically significant.

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