Blog Archives

Binary, beta, beta-binomial

September 10, 2018
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Binary, beta, beta-binomial

I’ve been working on updates for the simstudy package. In the past few weeks, a couple of folks independently reached out to me about generating correlated binary data. One user was not impressed by the copula algorithm that is already implemented. I’ve added an option to use an algorithm developed by Emrich and Piedmonte in 1991, and will be...

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The power of stepped-wedge designs

August 27, 2018
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The power of stepped-wedge designs

Just before heading out on vacation last month, I put up a post that purported to compare stepped-wedge study designs with more traditional cluster randomized trials. Either because I rushed or was just lazy, I didn’t exactly do what I set out to do. I did confirm that a multi-site randomized clinical trial can be more efficient than a...

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Multivariate ordinal categorical data generation

August 14, 2018
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Multivariate ordinal categorical data generation

An economist contacted me about the ability of simstudy to generate correlated ordinal categorical outcomes. He is trying to generate data as an aide to teaching cost-effectiveness analysis, and is hoping to simulate responses to a quality-of-life survey instrument, the EQ-5D. The particular instrument has five questions related to mobility, self-care, activities, pain, and anxiety. Each item has three...

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Randomize by, or within, cluster?

July 18, 2018
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Randomize by, or within, cluster?

I am involved with a stepped-wedge designed study that is exploring whether we can improve care for patients with end-stage disease who show up in the emergency room. The plan is to train nurses and physicians in palliative care. (A while ago, I described what the stepped wedge design is.) Under this design, 33 sites around the country will receive...

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How the odds ratio confounds: a brief study in a few colorful figures

July 9, 2018
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How the odds ratio confounds: a brief study in a few colorful figures

The odds ratio always confounds: while it may be constant across different groups or clusters, the risk ratios or risk differences across those groups may vary quite substantially. This makes it really hard to interpret an effect. And then there is inconsistency between marginal and conditional odds ratios, a topic I seem to be visiting frequently, most recently last...

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Re-referencing factor levels to estimate standard errors when there is interaction turns out to be a really simple solution

June 25, 2018
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Re-referencing factor levels to estimate standard errors when there is interaction turns out to be a really simple solution

Maybe this should be filed under topics that are so obvious that it is not worth writing about. But, I hate to let a good simulation just sit on my computer. I was recently working on a paper investigating the relationship of emotion knowledge (EK) in very young kids with academic performance a year or two later. The idea...

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Late anniversary edition redux: conditional vs marginal models for clustered data

June 12, 2018
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Late anniversary edition redux: conditional vs marginal models for clustered data

This afternoon, I was looking over some simulations I plan to use in an upcoming lecture on multilevel models. I created these examples a while ago, before I started this blog. But since it was just about a year ago that I first wrote about this topic (and started the blog), I thought I’d post this now to mark...

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A little function to help generate ICCs in simple clustered data

May 23, 2018
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A little function to help generate ICCs in simple clustered data

In health services research, experiments are often conducted at the provider or site level rather than the patient level. However, we might still be interested in the outcome at the patient level. For example, we could be interested in understanding the effect of a training program for physicians on their patients. It would be very difficult to randomize patients...

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Is non-inferiority on par with superiority?

May 13, 2018
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Is non-inferiority on par with superiority?

It is grant season around here (actually, it is pretty much always grant season), which means another series of problems to tackle. Even with the most straightforward study designs, there is almost always some interesting twist, or an approach that presents a subtle issue or two. In this case, the investigator wants compare two interventions, but doesn’t feel the...

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How efficient are multifactorial experiments?

May 1, 2018
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How efficient are multifactorial experiments?

I recently described why we might want to conduct a multi-factorial experiment, and I alluded to the fact that this approach can be quite efficient. It is efficient in the sense that it is possible to test simultaneously the impact of multiple interventions using an overall sample size that would be required to test a single intervention in a...

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