when the confidence level or P-value of a confidence interval or test of significance is accurate even when the assumptions of the procedure are violated
A distribution in which sigma is replaced with s. The t distribution is symmetric about zero and is bell-shaped, but the spread is greater because there is more variation
degrees of freedom
(n-1); defines a t distribution by indicating the number of values that the statistic can vary from the final calculation
1) SRS 2) sufficiently large population 3) approximately normal
when the standard deviation of a statistic is estimated from the data
matched pairs study
type of matched pairs design in which subjects are matched in pairs and the outcomes are compared within each matched pair
before and after study
design where randomization is not possible so matched pairs is used; ALWAYS "after minus before"
If the sample size is less than 15...
...use a normal probability plot and a modified boxplot.
If the sample size is at least 15...
...use a modified boxplot.
If the sample size is at least 40...
...you may use t procedures even if the data is skewed.
compares two treatments or two populations. The data consists of two independent SRSs, each drawn from a separate normally distributed population.
As degrees of freedom increase...
...the t distribution gets closer to the normal distribution, since s gets closer to sigma.