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ELI5: What is a p value and a null hypothesis in scientific research and how significant are they

I’m starting to get into science a lot more these days but I do not know what p values and null hypothesis are. 

Appreciate the help. Thank you.
gpt
Going to start off with some relevant definitions first 

Dependent variable — the thing that’s being measured, a response variable (ie. heart rate)

Independent variable — the thing that’s being manipulated or set by the researchers; the variable that is hypothesized to cause a change in the *dependent* variable (ie. a drug treatment) 

Population — the group being tested. Important to note that the conclusion can only be generalized to the population of the study. If the experiment (“sample population”) only includes men of Asian descent aged 45 and over, then the conclusion cannot be assumed to extend to men of other backgrounds, women, young men, children, etc. 

The alternative hypothesis is the research question in the form of a true/false statement (This drug affects heart rate). 

The null hypothesis is the “blank.” It assumes there is no relation between the independent and dependent variables (This drug has no effect on heart rate). With no evidence, we default that the null hypothesis is true. The experiment aims to disprove the null hypothesis in favor of the alternative.*

The p-value is the probability of getting the observed result under the assumption that the null hypothesis is true — that there is no relation between the variables.

A small p-value means that it would be very unlikely to observe this result by random chance; therefore, it is likely that something is causing it. In a well-designed experiment, the cause can be attributed to the independent variable.

*Just because a result is not significant, does not necessarily mean that the null hypothesis is definitively true, it just means we did not find evidence to say otherwise. Same goes for the alternative. Just because a result is significant, does not mean it is the end-all explanation. We just have evidence to support the conclusion. That’s not a go-ahead for all you conspiracy theorists out there to say “Gotcha!” If the results can be observed time and time again, then that’s more and more evidence to support the explanation.