A student asked me today about the differences between confounding and effect modification. In this post I’ll try and distinguish these conceptually and illustrate the differences using some very large simple simulated datasets in R.
Mixed model repeated measures (MMRM) in Stata, SAS and R
Linear mixed models are a popular modelling approach for longitudinal or repeated measures data. They extend standard linear regression models through the introduction of random effects and/or correlated residual errors. In the context of randomised trials which repeatedly measure patients over time, linear mixed models are a popular approach of analysis, not least because they handle missing data in the outcome ‘automatically’, under the missing at random assumption. Because of this a mixed model analysis has in many cases become the default method of analysis in clinical trials with a repeatedly measured outcome.
What might the true sensitivity be for lateral flow Covid-19 tests?
Disclaimer: I am in absolutely no way an expert on Covid-19 or tests for it. This post was motivated from undergraduate teaching for medical statistics, and probably makes assumptions which are implausible.
This morning I spoke to my undergraduate medical statistics students briefly about this piece in the BMJ published a few days ago raising concerns about the possible low test sensitivity of the rapid lateral flow tests which are being used in various contexts currently in the UK. This piece cites a UK government document which says concerning the lateral flow tests that:
Results of the PHE and Oxford University Innova evaluation show it has an overall analytical sensitivity of 76.8% for all PCR-positive individuals but detects over 90% of individuals with high viral loads
https://www.gov.uk/government/publications/community-testing-explainer/community-testing-a-guide-for-local-delivery#what-the-community-testing-programme-is
and that
In field evaluations, such as Liverpool, these tests still perform effectively and detect at least 50% of all PCR positive individuals and more than 70% of individuals with higher viral loads in both symptomatic and asymptomatic individuals.
https://www.gov.uk/government/publications/community-testing-explainer/community-testing-a-guide-for-local-delivery#what-the-community-testing-programme-is