The experience of Altmetric LLP, an altmetric tool developer, reveals common issues thatdemand attention when designing alternative metrics for response to scholarly writings.Identifying what can and should be measured for different user groups is fundamental. Adefault is to count all relevant mentions in a set of online sources, permitting drill down formore qualitative information. Data source selection varies by need, ranging fromgovernment documents to social media comment sites. Since the topic of discussion canbe elusive, a tracking method must point backward to original articles or data. Text mininghelps for text documents, but audio and video are less workable. Multiple versions of asingle article and subsections of books and datasets add ambiguity and redundancy. Validinterpretation depends on context and the relevance and timeliness of data and sources,requiring continual reassessment.