When you read, your brain does two things at once. It processes new information (ideas, claims, details you haven't seen yet) and reinforcement (repetition, callbacks, anchoring to what's already established).
Good writing balances these. Give the reader something new, then anchor it. Something new, anchor it. Like breathing. When the rhythm works, you don't notice it. You just keep reading.
Bad writing breaks this rhythm in two ways:
OVERLOAD: Too many new concepts too fast. Your working memory maxes out. You feel lost. You leave.
REPETITION: The same ideas restated with the same words. Nothing new arrives. You feel bored. You leave.
This tool measures that rhythm mathematically. It slides a window across your text and at each position computes how much is new (Expansion) versus how much is repeated (Compression). The ratio between them is the Stability Index (S).
S near zero = balanced. Reader stays engaged.
S spikes positive = overloaded. Reader is overwhelmed.
S spikes negative = repetitive. Reader is bored.
The science behind this spans five independent research fields: cognitive load theory (Sweller, 1988), redundancy research (Mayer, 2001), Shannon information theory (1948), lexical diversity studies (Johnson, 1944+), and the Coh-Metrix engagement research (Graesser & McNamara, 2004-2014). All five converge on the same finding: the balance between novelty and reinforcement determines whether people keep reading.
This is the first tool that unifies those findings into a single measurement.