Variation, Variability, Batches and Bias in Microarray Experiments: An Introduction

Microarray-based measurement of gene expression levels is a widely used technology in biological and medical research. The discussion around the impact of variability on the reproducibility of microarray data has captured the imagination of researchers ever since the invention of microarray technology in the mid 1990s. Variability has many sources of the most diverse kinds, and depending on the experimental performance it can manifest itself as a random factor or as a systematic factor, termed bias. Knowledge of the biological/medical as well as the practical background of a planned microarray experiment helps alleviate the impact of systematic sources of variability, but can hardly address random effects.