18
votes
Accepted
Causal diagrams necessary in randomized controlled trials?
In short, yes there are cases where one may want to draw a DAG. I will offer a simplified example.
As you mention, (proper) randomization ensures there are no confounders so we don't need to worry ...
- 39.6k
16
votes
Causal diagrams necessary in randomized controlled trials?
Classic randomized clinical trial analysis uses the intention to treat approach in which all patients are analyzed and they are analyzed using the randomization codes. Secondary analyses may require ...
- 98.4k
16
votes
Why should you want to use a smaller control group?
Power-wise, yes, a 1:1 comparison is going to be statistically optimal. There are several reasons why not all studies use balanced allocation however, by no means exhaustive:
The idea of being ...
- 5,683
11
votes
Why is the Central Limit applicable in A/B testing?
Suppose you have two populations: A and B.
You draw samples from A: $a_1,a_2,...,a_n$
You draw samples from B: $b_1,b_2,...,b_n$
The actual values of $a$'s and $b$'s are just the numbers $0$'s and $1$'...
- 3,213
11
votes
Why should you want to use a smaller control group?
I don't know where you read this, so I don't know what their reason was, but one possible reason is ethical, rather than statistical.
If you expect the effect size to be large, then you will not need ...
- 128k
9
votes
Are the arguments that placebo can be unethical mistaken?
A randomized controlled trial requires that there is a sincere uncertainty about the relative efficacy of the treatments under investigation. The term for this uncertainty is "equipoise."
...
- 2,201
8
votes
Switching interventions after randomization
Don't.
The whole point of randomization is that it removes the need to make assumptions like assuming location isn't associated with the outcome. Yes, certainly, you can do it, but your confidence in ...
- 9,923
8
votes
Causal diagrams necessary in randomized controlled trials?
Even in an RCT, a DAG can be useful to examine colliders and mediators
In general, there are two types of variables that might might adversely impact an RCT and which may require consideration and ...
- 133k
7
votes
Objections to randomization
The papers from Koch, Abel, and Urbach do not reject randomization summarily as a means to achieve 1-4, rather they claim it is neither sufficient nor necessary to achieve those criteria. The take-...
- 64.8k
7
votes
Accepted
weighted randomization
The book Randomization in Clinical Trials (2nd ed.) by William Rosenberger and John Lachin (2016, Wiley) explains several methods for randomization for balancing treatment assignments (chapter 3). In ...
- 31.2k
7
votes
Why is the Central Limit applicable in A/B testing?
we only draw two samples
You can consider a sample of size $n$ as $n$ samples of size $1$.
The outcome can be seen as a sum of $n$ independent Bernoulli distributed variables (if the people in the ...
- 86.5k
7
votes
Accepted
What is a Randomized Complete Block Design. How do we create one and analyze it?
There are a number of different designs that use the term "Randomised Complete Block Design", but they are all based on a very basic idea, that of blocking. A block is just another factor (a ...
- 65.8k
7
votes
Accepted
When and how of stratification?
This is a big topic, but a few points for starters.
Q1 "When is this necessary? (Aka Shouldn't pure randomization get "close enough"?)"
Stratification is really only done for two ...
- 2,701
6
votes
Can a truly randomized procedure (e.g. random treatment allocation) result in unbalanced distributions?
In the following answer, I'm going to discuss imbalances between the number of subjects allocated to the groups. If you're instead interested in imbalances in covariates, see myth 5 in Senn (2013).
...
- 31.2k
6
votes
Accepted
What is a "split-body" RCT?
It's a trial whether the two interventions are applied to different parts of the same person. One setting is ophthalmology, with different treatments applied to each eye, but this is an example with ...
- 46.8k
6
votes
Accepted
Switching interventions after randomization
Manual intervention of switching the randomization labels would typically be frowned upon, but should have no statistical effect. A statistically valid randomization between two groups is no less ...
- 10.4k
6
votes
Accepted
How should I analyse this experiment with two blocking factors
Is this another type of Randomized Complete Block Design?
It is very closely related but this seems to be a Latin Square design, not a RCB design, though we could say that it's an extension of the ...
- 65.8k
6
votes
Accepted
Analyzing an experiment which consists of many "small" experiments
This is a standard A/B test if you view cakes as the sample from a population of products (rather than customers), with a varying number of taste tests (aka trials) for each cake.
Your proposed ...
- 38.3k
5
votes
Accepted
Is it worth redoing random split several times in order to draw the 'best' control group?
I would say that your customer is right.
For my rationale, let's discuss why we do random assignment to groups in the first place. In an ideal world, each individual in the TG would correspond to ...
- 131k
5
votes
Sample size calculation for a multicenter RCT
Given your non-trivial design, I would suggest you turn this task a bit onto its head and use a simulation-based sample size planning approach. A nice intro to this would be: External validation of ...
- 46k
4
votes
Accepted
Systematic allocation of treatment: is there a chance of biasing a design?
If the randomization sequence is discovered, patients could be non randomly assigned to the experimental group of choice. In a truly blinded study, this is unlikely. In many studies, blinding may not ...
- 2,201
4
votes
Accepted
Advantages/disadvantages of fractional factorial design vs completely randomized design
What you propose is to randomize the choice of treatments. That is not a completely randomized design (CRD), a CRD is about randomizing the allocation of experimental units to the treatments.
Good ...
- 82.8k
4
votes
Accepted
Is it possible to to have more than 2 groups for biased coin randomization? If no, is there any modifications supporting multiple groups?
Efron's biased coin design is predicated on two groups. From the error message you're getting, it seems like the R function you're using has not been extended to more than 2 groups. Proposing an ...
- 2,220
4
votes
How to check if randomization was proper
The best way to generate a "reproducible randomisation" is to use a scripted randomisation algorithm in a statistical programming language and make sure you "set the seed" for the ...
- 133k
4
votes
Block design or completely randomized?
In your case, the "treatment" is the condition that you assign to the subjects at random. As Bruce explained, this is simply a randomized assignment of the treatment but not a blocking ...
4
votes
Should AstraZeneca's results be discounted?
My opinion is no. I felt that AZ's primary publication was high quality: reporting the planned analyses, presenting a very important unplanned analysis, explaining the unexpected results of that ...
- 64.8k
4
votes
Using PCA to combine variables in a randomized trial with a baseline and a follow-up measurement
The method of collapsing SBP and DBP into one measure should be guided by subject matter expertise. If you are interested in MAP then compute MAP. PCA is guided by variance maximization not by ...
- 98.4k
4
votes
In a randomized trial, should we exclude random intercepts and use only slopes?
The standard model for this that is commonly used in randomized controlled trials (RCTs) is ...
- 35.2k
4
votes
Accepted
Sample size calculation for a multicenter RCT
Most statisticians ignore centers when doing sample size calculations. That doesn't cause too much of a problem.
The best models for duration of treatment are the Cox proportional hazards model or ...
- 98.4k
4
votes
Why should you want to use a smaller control group?
With multiple treatment arms that are being compared to a single control arm, the logic is a bit different than usually. In this situation, not a 1:1:...:1 allocation, but the square root allocation $...
- 1,909
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