In a manner involving the measurement of pain; using instruments or methods designed to quantify pain.
From algometrical + -ly adverbial suffix. This highly technical adverb follows standard English word-formation rules to convert the adjective into its adverbial form.
This is the kind of word you'd only encounter in dense medical research papers, where researchers need to specify that they measured pain in a standardized way—it's technical language that actually helps other scientists understand exactly what was done.
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