exm 3 Flashcards
what are the properties of a normal distribution
continuous, symmetrical around mean, single mode, density highest at mean
what are the 2 rules of thumb for normal distributions
2/3 of area under the curve lies within one sd of mean
2. 95% of the area under curve lies within 2 sd deviations of mean
observational studies
-easier
-cant be randomized
-can establish association, but cannot determine cause of association
experimental studies
-permits random assignment
-can determine causes of association
-minimize influence of cofounding variables
what are the 3 ways to reduce bias
simultaneous control group, randomization, blinding
how to reduce effects of sampling error
replication, balance, blocking
central limit theorem
sampling distribution of mean will always be normally distributed as long as sample size is large enough
z table is critical value so if 2 *p = <0.05 then we
we reject the null
how to calculate standard error for a normal population
s/sqrt of sample size
why should you use t distribution instead of Z
important fro confidence intervals or hypothesis due to sampling error in estimate of SE
t distribution versus Z
t has fatter tails and a lower peak due to sampling error
what does normal distribution describe
sampling distribution of many estimates
assumptions of one-sample t test
must be a random sample from population, variable must have a normal distribution in population
paired versus two independent sample
2 sample- measurements are independent
paired- more powerful- less variation
how to deal with violation of assumptions
ignore, transform, non-parametric tests, permutation tests
what are the ways to detect deviations from normality
graphical methods- histogram, normal quantile plot
formal test- shapiro wilks
why do you need to be cautious about interpreting shapiro-wilk test results for small sample sizes and for large sample sizes
if sample size is too low- power is low.
as n gets larger assumption of normality becomes less important
when to ignore violations
when n>30 and sd is less than 3
