Rate measurements used to determine a rate law are usually taken within the first few percent of reaction. Why favor such early readings?

Core Concept

A rate law is defined in terms of reactant concentrations, which are most reliably known at the start before any depletion. Products have barely accumulated, so the reverse process contributes essentially nothing to the net rate. Later readings blend forward and reverse contributions with constantly drifting concentrations, muddying any comparison between rate and concentration. Initial rates therefore provide the cleanest data for extracting orders.