Biostatistics Topic 6

p-value

probability of an observed (or more extreme) result arising by chance, assuming the null hypothesis is true

null distribution

sampling distribution of all possible values of the test statistics that assumes the null hypothesis is true

test statistic

measures how far the sample data are from the result expected under the null hypothesis

closer to the tail = more unusual =

smaller probability

what is the null hypothesis and the alternative hypothesis of a two sided test

H0: p=0.5 and HA: p not = 0.5

what must you include when reporting the result of a statistical test

test statistic (magnitude of the difference between observed data and expectation of the null hypothesis) and the exact P-value

statistical decision

where you state whether you reject or fail to reject the null hypothesis

how do you describe what a p-value is

remember that the p-value is related to the test statistic from the sample and you need to explain what the value represents - a probability

what are the three criteria for the description of the p-value

probability, data observed in the sample or observations taht would be more unusual and assuming that the null hypothesis is true

in biology what do you set to make a statistical decision

significance level (alpha)

what is 0.05 used to identify

a cut off (threshold) to determine statistical significance

what is the cut off point called

critical value

when would we reject the null hypothesis

if the P-value is below 0.05

why 0.05

5% mark is low enough that we can reject the null hypothesis beyond most reasonable doubts

rejecting a true null is a what

type I error

what happens when you have a type I error

detecting a difference with your sample when in reality no difference exists

what happens when you fail to reject a false null

type II eror

what happens during a type II error

in reality a difference exists, but we don’t detect it with our sample

hypothesis testing

compares data to what we would expect to see if a specific null hypothesis was true

null hypothesis

specific statement abotu a population parameter made for the purpose of argument

alternative hypothesis

includes all other feasible values for the population parameter besides the value stated in the null hypothesis

two-tail

the alternative hypothesis includes parameter values on both sides of the parameter values specified by the null hypothesis

power

probability that a random sample will lead to rejection of a false null hypothesis