Multimodal Outcomes in N-of-1 Trials: Combining Unsupervised Learning and Statistical Inference
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N-of-1 trials are randomized multi-crossover trials in single participants with the purpose of investigating the possible effects of one or more treatments. Research in the field of N-of-1 trials has primarily focused on scalar outcomes. However, with the increasing use of digital technologies, we propose to adapt this design to multimodal outcomes, such as audio, video, or image data or also sensor measurements, that can easily be collected by the trial participants on their personal mobile devices. We present here a fully automated approach for analyzing multimodal N-of-1 trials by combining unsupervised deep learning models with statistical inference. First, we train an autoencoder on all images across all patients to create a lower-dimensional embedding. In the second step, the embeddings are reduced to a single dimension by projecting on the first principal component, again using all images. Finally, we test on an individual level whether treatment and non-treatment periods differ with respect to the component. We apply our proposed approach to a published series of multimodal N-of-1 trials of 5 participants who tested the effect of creams on acne captured through images over 16 days. We compare several parametric and non-parametric statistical tests, and we also compare the results to an expert analysis that rates the pictures directly with respect to their acne severity and applies a t-test on these scores. The results indicate a treatment effect for one individual in the expert analysis. This effect was replicated with the proposed unsupervised pipeline. In summary, our proposed approach enables the use of novel data types in N-of-1 trials while avoiding the need for manual labels. We anticipate that this can be the basis for further explorations of valid and interpretable approaches and their application in clinical multimodal N-of-1 trials.
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