Effect Modification (Interaction) Flashcards
What is a modifier (Z)?
A variable that alters the relationship between the X1 and Y
What is the interaction term?
The cross-product between X1 and the modifier
X1 x Z
If β3 has a p<0.05, what can you conclude about the modification?
There is a significant effect modification.
There is a significant interaction between X1 and the modifier Z
What are the main effects of the interaction interpretation?
β1 and β2
(effect of X1 on Y when Z=0, and effect of Z on Y when X1=0)
If there is an interaction, how do you calculate the effect of X1 on Y?
β1 + β3 x Z
(Z = 0 or 1 if coded, use average if continuous)
How do you interpret a negative interaction effect?
Effect of one variable decreases as the other variable increases
How can you present continuous x continuous interactions?
▪️Table with quartiles
▪️Graph plotting quartiles
What is an outlier?
An observation that lies an abnormal distance from other values in a random sample
What are influential observations?
Outliers with large influence on the fitted regression model
May distort the actual relationship
How can you spot outliers in your dataset?
▪️Sort by ascending/descending
▪️Look at minimums and maximums
▪️Plot on graph e.g. regression variable plots
What are mild outliers?
Outliers that lie between 1.5 and 3 times the IQR (below Q1 or above Q3)
(between inner and outer limits)
What are extreme outliers?
Outliers that lie more than 3 times the IQR (below Q1 or above Q3)
(beyond outer limit)
What is the lower outer fence?
Q1 - (3 x IQR)
What is the lower inner fence?
Q1 - (1.5 x IQR)
How can you find outliers using standardised residuals?
Any observation with an absolute standardised residual larger than 3
