Biostats Quiz 4!

what does ANOVA stand for

analysis of variance

underline method strat

groups should be ordered from largest to smallest

how to calculate degrees of freedom

• one sample: n-1

• paired: n-1

• two sample: N-2

• none for Mann Whitney U because it is nonparametric. Degrees of freedom involves estimating parameters

graph for 1 nominal and 1 continuos variable

bar graph with standard error

degrees of freedom in 2 sample t test

N-2

standard deviation

 typical deviation of data from the mean, has to do with spread

what letter distribution does ANOVA use? Turkey? Mann Whitney U?

F

q

U

what does F statistic/distribution tell you

ratio of between-group variance to within-group variance

MS

the average variance. It measures the variances between the different samples, and can be done in respect to model or error.

criteria for ANOVA

Parametric, normal distribution, 3+ quantitative samples

Nominal x cont. Y

anova is a ___ analysis of difference

one way, because it doesn’t tell you what group mean is statistically different from the others, given the null is rejected. AND BECAUSE IT IS ONE TAILED

2 forms of ANOVA hypotheses

Conceptual: the normal one we do

Statistical: compares model vs. error variance, tells you if there is more signal than noise

assumptions of ANOVA (4)

• Representative samples

• Normally distributed data from all samples

• Data is all independent within and between samples

• Homogeneity of variances (variance between groups is all the same)

grand mean

overall average of data across all groups

is variance or sum of squares additive?

SS, adding variances inflates variability

model:

variation explained by the independent variable, reflects how different the groups are from each other, is the “signal”

error

reflects unexplained variation within groups, is the “noise”

I/K

number of groups in ANOVA

F ratio

a ratio of variance/mean squares, decide whether group means are actually different or just different due to randomness. If it is much larger than one there is likely significant difference

ratio of signal over noise. If there is more signal than noise we know at least one group is different

sum of squares/what do the different kinds tell you

• measure of total variation, tells you how spread out the data is by adding up squared differences. 

  • Model: variation explained by group differences, how far group mean is from overall mean

  • Error: unexplained variation, how far each individual value if from its group mean

  • Total: total variation in all data

2 sample t test modeling type

Nominal x, cont. Y

how to calculate a confidence interval

use formula, for two sample t tests you collect it for the difference between two population means (M1-M2, instead of just M like a one sample).

Ex. I am 95% confident that the difference between population mean 1 and 2 lies between these values.

what is important about the sign of t in a two sample t test

• When you load confidence, make sure of the sign of your t matches your hypotheses (ex. If cocaine increases running time as opposed to saline, ensure your t will be positive given the alt. Is true)

  • Or if medication decreases depression scores, your t will be negative given the alt. Is true

  • Be careful that if you don't load confidence your t value is both + and -

strat for Whitney U

Always use the larger U value (so pick the smaller between R1 and R2)

what does the q value tell us

In tukey test, tells us how far apart two group means are relative to the variability in your data.

conclusion for two sample t test vs Whittney

• two sample t test can give you conclusions about parameters, Whitney cannot! Don’t talk about average/mean in your Mann Whitney conclusion

how to find Fcv

you don’t find it idiot, if your ANOVA is significant you use the q distribution