RNA SHAPE experiments have become important and successful sources of information for RNA structure prediction. In such experiments, chemical reagents are used to probe RNA backbone flexibility at the nucleotide level, which in turn provides information on base pairing and therefore secondary structure. Little is known, however, about the statistics of such SHAPE data. In this work, we explore different representations of noise in SHAPE data and propose a statistically sound framework for extracting reliable reactivity information from multiple SHAPE replicates. Our analyses of RNA SHAPE experiments underscore that a normal noise model is not adequate to represent their data. We propose instead a log-normal representation of noise and discuss its relevance.Under this assumption, we observe that processing simulated SHAPE data by directly averaging different replicates leads to bias. Such bias can be reduced by analyzing the data following a log transformation, either by log-averaging or Kalman filtering. Application of Kalman filtering has the additional advantage that a prior on the nucleotide reactivities can be introduced. We show that the performance of Kalman filtering is then directly dependent on the quality of that prior. We conclude the paper with guidelines on signal processing of RNA SHAPE data. September 4, 2018 1/31 Introduction 1 Beyond its role in protein synthesis and the transfer of genetic information, RNA exists 2 as a dynamic cellular component at the core of gene regulation [1]. From microRNAs 3 involved in regulating gene expression [2] and long noncoding RNAs similarly regulating 4 gene expression [3] to ribozymes acting as chemical catalysts [4], RNA plays a central 5 role in a multitude of cellular activities. The diverse repertoire of biological functions 6 that RNAs adopt is deeply rooted in their abilities to form complex three-dimensional 7 structures [1]. This interplay between structure and function underscores the need for 8 robust structural analysis as a prerequisite to a full understanding of the physiological 9 role of RNA [5]. Despite its importance, determining the complex 3D structures of RNA 10 remains a challenging problem, particularly for longer RNAs [6, 7]. 11 Considering the hierarchical nature of RNA folding [8], much of the efforts in 12 structure determination have been devoted to its two-dimensional base-pairing pattern, 13 also known as its secondary structure. This secondary structure is generally considered 14 to be more stable than and independent of the final 3D conformation [8]. Though 15 experimental methods such as nuclear magnetic resonance (NMR) [9] and 16 crystallography [10] can be used to accurately resolve 3D RNA structures, they are 17 time-consuming, expensive, and often preclude the analysis of long or flexible 18 molecules [11]. Comparative sequence analysis, the process of inferring base-pairing 19from co-variations observed in the alignment of homologous sequences, is a robust 20 method for defining the secondary structure of RNA [11,12]. Howeve...