Advanced Research Methods Flashcards
Definition DAG
Directed Acyclic Graphs are graphical representations of the causal structure underlying a research question.
DAGs help to visualize the causal structure underlying a research question
What do you need for a DAG?
- Prior knowledge of the subject
- Data on all relevant variables
Path
Any route between exposure X and outcome Y
connection between exposure and outcome
Causal path
Follows the direction of the arrows
Backdoor Path
Does not follow the direction of the arrows
Open paths
All paths are open, unless they collide somewhere on a path
Closed paths
A path is closed if arrows collide in one variable on that path
When is an open path blocked?
When adjusting for a variable
What do we want to know from an causal inference?
We are not interested in the outcome per se, we are interested in the role of the treatment in achieving this outcome
Definition causal effect
In an individual, a treatment has a causal effect if the outcome under treatment 1 would be different from the outcome under treatment 2
Counterfactual outcome
Potential outcome that is not observed because the subject did not experience the treatment
Individual causal effect cannot be observed unless..
Except under extremely strong and generally unreasonable assumptions
When can a causal inference be determined?
Only when three identifiability conditions are met in a study
The three identifiability conditions
Positivity
Consistency
Exchangeability
If all conditions are met the association between exposure and outcome is an unbiased estimate of a causal effect
Positivity
Each individual has to have a positive probability of being assigned to each treatment arms
Consistency
The treatment has to be well defined
Exchangeability
- The individuals assigned to the different treatment arms have to be similar
- It does not matter who gets treatment A and who gets treatment B
How to meet the exchangeability condition
- randomized rct
Individuals are randomly assigned to one of each treatment - Matching
For each individual who gets treatment A, there is an individual who gets treatment B - Stratification
Randomly select individuals from different subsets of the larger population. Almost impossible - Adjustment
Control for factors that influence the association between the treatment and outcome
Confounder
An variable that effects X and Y
Ethnography
The task is to document the culture, the perspectives and practices, of the people in the settings. The aim is to get inside the way each group of people sees the world
Correlation
A statistical relationship between the treatment and outcome
relative risk or risk ratio
the probability of an outcome in an exposed group to the probability of an outcome in an unexposed group
RR = 1 exposure does not affect outcome
RR < 1 the risk of the outcome is decreased by the exposure
RR > 1 the risk of the outcome is increased by the exposure
Risk difference
The difference between the risk of an outcome in the exposed group and the unexposed group.
Absolute risk increase
When the risk of an outcome is increased by the exposure
odds ratio
is a statistic that quantifies the strengts of the association between two events.
OR = 1 A and B are independent
OR > 1 A and B are associated correlation
OR < 1 A and B are negatively correlated
confounding
bias caused by common cause of exposure and outcome
You have to include and control/adjust the variable
collider
Variable where two arrows collide. The variable has to be excluded
Blocking
adjusting for a variable amongst a path. Blocking can be done by adjusting for any variable along a path.
Unblocking
Adjusting for a collider, unblocking a path by adjusting for an already blocked path
selection bias
if there is no equal chance of person a or person b becoming part of the sample
publication bias
positive findings are more likely to be published, which can skew the results that we see
mediator
explains the relation between the independent and the dependent variable. It explains how or why there is a relation
moderator
is a variable that effects the strength of the relation between the predictor and the criterion variable
self selection bias
when individuals volunteer to be in a treatment group. The sample in not random
recall bias
systematic error that occurs when participants do not remember previous events omit details
survival ship bias
when some of many of the observations are falling out of the sample which changes the composition of observations that are left
healthy user bias
people who take vitamins regularly are likely to be healthy
omitted variables bias
variables are neglected that may be important in the relationship
regression equation
example:
Weight = B0 + B1 x heigh
B0= constant
p <0,05
difference is statistically significant
the chance of finding a statistically significant result depends on
- sample size
- variation in population
testing
gives dichotomous result yes/no
estimating
size/strength of estimated effect
interpretation 95% CI
if the study was repeated, 95% of intervals would contain correct value
data ministry
adding too many variables without any theoretical justitification
multicollinearity
highly correlated explanatory variables
extrapolating beyond the data
regression results are only valid for populations similar to that of the study sample